Fix rare double free in MUPIP BACKUP in case of MUPIP STOP

Background
----------
* In internal testing, we noticed a rare failure in the `v51000/mu_bkup_stop` subtest
  where a `mupip backup` process that was sent a `SIGTERM` (by the test) ended up
  creating a core file due to ASAN assert failing on a double free.

* Below are relevant details from the core file.

  ```c
  Core was generated by `mupip backup -online -dbg * ./49181_online1'.
  Program terminated with signal SIGSEGV, Segmentation fault.

  (gdb) where
  #0  ydb_os_signal_handler (sig=11, info=0x7fd09968c3f0, context=0x7fd09968c2c0) at sr_unix/ydb_os_signal_handler.c:57
  #1  <signal handler called>
  #2  ydb_os_signal_handler (sig=6, info=0x7fd09968caf0, context=0x7fd09968c9c0) at sr_unix/ydb_os_signal_handler.c:57
  #3  <signal handler called>
  #4  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at ./nptl/pthread_kill.c:44
  #5  __pthread_kill_internal (signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:78
  #6  __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
  #7  __GI_abort () at ./stdlib/abort.c:79
  #8  __sanitizer::Abort () at ../../../../src/libsanitizer/sanitizer_common/sanitizer_posix_libcdep.cpp:143
  #9  __sanitizer::Die () at ../../../../src/libsanitizer/sanitizer_common/sanitizer_termination.cpp:58
  #10 __asan::ScopedInErrorReport::~ScopedInErrorReport (this=0x7ffda6de6ebe, __in_chrg=<optimized out>) at ../../../../src/libsanitizer/asan/asan_report.cpp:190
  #11 __asan::ReportDoubleFree (addr=140533757257728, free_stack=<optimized out>) at ../../../../src/libsanitizer/asan/asan_report.cpp:224
  #12 __asan::Allocator::ReportInvalidFree (this=<optimized out>, stack=0x7ffda6de79f0, chunk_state=<optimized out>, ptr=0x7fd090ae2800) at ../../../../src/libsanitizer/asan/asan_allocator.cpp:757
  #13 __interceptor_free (ptr=0x7fd090ae2800) at ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:53
  #14 system_free (addr=0x7fd090ae2800) at sr_port/gtm_malloc_src.h:1485
  #15 gtm_free_main (addr=0x7fd090ae2800, stack_level=1) at sr_port/gtm_malloc_src.h:854
  #16 gtm_free (addr=0x7fd090ae2800) at sr_port/gtm_malloc_src.h:1501
  #17 mubclnup (curr_ptr=0x0, stage=need_to_del_tempfile) at sr_port/mubclnup.c:103
  #18 mupip_backup_call_on_signal () at sr_port/mupip_backup.c:208
  #19 signal_exit_handler (exit_handler_name=0x7fd097f1dda0 "deferred_exit_handler", sig=15, info=0x7fd098480fd8 <stapi_signal_handler_oscontext+3320>, context=0x7fd098481058 <stapi_signal_handler_oscontext+3448>, is_deferred_exit=1) at sr_unix/signal_exit_handler.c:67
  #20 deferred_exit_handler () at sr_unix/deferred_exit_handler.c:120
  #21 deferred_signal_handler () at sr_port/deferred_signal_handler.c:95
  #22 system_free (addr=0x7fd090ae2800) at sr_port/gtm_malloc_src.h:1486
  #23 gtm_free_main (addr=0x7fd090ae2800, stack_level=1) at sr_port/gtm_malloc_src.h:854
  #24 gtm_free (addr=0x7fd090ae2800) at sr_port/gtm_malloc_src.h:1501
  #25 mubclnup (curr_ptr=0x0, stage=need_to_del_tempfile) at sr_port/mubclnup.c:103
  #26 mupip_backup () at sr_port/mupip_backup.c:1585
  #27 mupip_main (argc=6, argv=0x7ffda6deef18, envp=0x7ffda6deef50) at sr_unix/mupip_main.c:130
  #28 dlopen_libyottadb (argc=6, argv=0x7ffda6deef18, envp=0x7ffda6deef50, main_func=0x55af49fd9020 "mupip_main") at sr_unix/dlopen_libyottadb.c:151
  #29 main (argc=6, argv=0x7ffda6deef18, envp=0x7ffda6deef50) at sr_unix/mupip.c:21

  (gdb) f 25
  #25 mubclnup (curr_ptr=0x0, stage=need_to_del_tempfile) at sr_port/mubclnup.c:103
  103                                     free(ptr->backup_hdr);

  (gdb) f 17
  #17 mubclnup (curr_ptr=0x0, stage=need_to_del_tempfile) at sr_port/mubclnup.c:103
  103                                     free(ptr->backup_hdr);

  (gdb) down
  #24 gtm_free (addr=0x7fd090ae2800) at sr_port/gtm_malloc_src.h:1501
  1501            gtm_free_main(addr, TAIL_CALL_LEVEL);

  (gdb) down
  #23 gtm_free_main (addr=0x7fd090ae2800, stack_level=1) at sr_port/gtm_malloc_src.h:854
  854                     system_free(addr);

  (gdb) down
  #22 system_free (addr=0x7fd090ae2800) at sr_port/gtm_malloc_src.h:1486
  1486            ENABLE_INTERRUPTS(INTRPT_IN_FUNC_WITH_MALLOC, prev_intrpt_state);

  (gdb) list
  1481    {
  1482            intrpt_state_t  prev_intrpt_state;
  1483
  1484            DEFER_INTERRUPTS(INTRPT_IN_FUNC_WITH_MALLOC, prev_intrpt_state);
  1485            free(addr);
  1486            ENABLE_INTERRUPTS(INTRPT_IN_FUNC_WITH_MALLOC, prev_intrpt_state);
  1487            return;
  1488    }
  ```

Issue
-----
* We did a `free(ptr->backup_hdr)` at line 103. And that in turn ended up using the system `free()`
  function because the test framework had randomly set the `gtmdbglvl` env var to a value of
  `0x80000000`.

* So at line 1485 above, the system free finished but at line 1486 we noticed the SIGTERM that was
  deferred and so decided to handle it. But the `ptr->backup_hdr` variable was still set to a
  non-NULL value so as part of the deferred exit handler, we tried to free this again resulting
  in the double free.

Fix
---
* The fix is to note `ptr->backup_hdr` in a local variable and clear the former and then attempting
  the `free()` on the local variable. This way if we decide to do deferred exit handling after the
  `free()` occurred, we will notice a NULL value of `ptr->backup_hdr` and so avoid the double free.

Notes
-----
* This is considered a too rare a race condition to be encountered in practice and so it is expected
  to not be noticed by a user. Therefore no YDB issue is created for this.

sr_port/mubclnup.c

@@ -3,7 +3,7 @@
  * Copyright (c) 2001-2021 Fidelity National Information	*
  * Services, Inc. and/or its subsidiaries. All rights reserved.	*
  *								*
- * Copyright (c) 2023 YottaDB LLC and/or its subsidiaries.	*
+ * Copyright (c) 2023-2024 YottaDB LLC and/or its subsidiaries.	*
  * All rights reserved.						*
  *								*
  *	This source code contains the intellectual property	*
@@ -100,7 +100,16 @@ void mubclnup(backup_reg_list *curr_ptr, clnup_stage stage)
 				|| (give_up_after_create_tempfile == ptr->not_this_time));
 			if (give_up_before_create_tempfile != ptr->not_this_time)
 			{
-				free(ptr->backup_hdr);
+				/* Need to free "ptr->backup_hdr". Use temporary variable to avoid double-free in
+				 * case we get a signal (e.g. SIGTERM during the "free") as the deferred signal handler
+				 * would then notice a non-NULL value of "ptr->backup_hdr" and try to free it again
+				 * even though it was already freed before the signal/interrupt.
+				 */
+				sgmnt_data_ptr_t	tmp_hdr;
+
+				tmp_hdr = ptr->backup_hdr;
+				ptr->backup_hdr = NULL;
+				free(tmp_hdr);
 				if (online)
 				{	/* Stop temporary file from growing if we made it active */
 					if (keep_going == ptr->not_this_time)