Fix crash and memory errors reported by valgrind for v2.1.1

[JavierJF]

• A clear description of the issue

After a crash detected in the CI with backtrace:

(gdb) bt
#0  __memmove_avx_unaligned_erms () at ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:500
#1  0x0000563709c468c1 in memcpy (__len=16777215, __src=<optimized out>, __dest=<optimized out>) at /usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
#2  MySQL_ResultSet::add_row2 (this=this@entry=0x7fc94fa2b000, row=row@entry=0x7fc8a4f77bd8, offset=<optimized out>,
    offset@entry=0x7fc753200540 "\377\377\377\r") at MySQL_Protocol.cpp:2852
#3  0x0000563709c49c73 in MySQL_ResultSet::init_with_stmt (this=this@entry=0x7fc94fa2b000) at MySQL_Protocol.cpp:2727
#4  0x0000563709c25a0d in MySQL_Session::MySQL_Stmt_Result_to_MySQL_wire (this=0x7fc94fa3a200, stmt=0x7fc94fa0e800, myconn=0x7fc94fa39f00)
    at MySQL_Session.cpp:6344
#5  0x0000563709c26b85 in MySQL_Session::handler_rc0_PROCESSING_STMT_EXECUTE (this=this@entry=0x7fc94fa3a200, myds=myds@entry=0x7fc94fa48c00)
    at MySQL_Session.cpp:3878
#6  0x0000563709c3c20d in MySQL_Session::handler (this=this@entry=0x7fc94fa3a200) at MySQL_Session.cpp:4459
#7  0x0000563709c15a4c in MySQL_Thread::process_all_sessions (this=this@entry=0x7fc94fa00000) at MySQL_Thread.cpp:4883
#8  0x0000563709c1d8a8 in MySQL_Thread::run (this=0x7fc94fa00000) at MySQL_Thread.cpp:4417
#9  0x0000563709bbf7c4 in mysql_worker_thread_func (arg=0x7fc9532dc310) at main.cpp:769
#10 0x00007fc953f51609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#11 0x00007fc953b25103 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Full coredump from this crash omitted due to size

A research was conducted to get the cause of the failure, a fast inspection of the dump pointed out to the recent changes required to 'ps_buffer' (see #3324 for more context into previous related issue):

(gdb) p *MyRS->stmt
$3 = {mem_root = {free = 0x7fc94fa02800, used = 0x0, pre_alloc = 0x7fc94fa02800, min_malloc = 32, block_size = 2024, block_num = 4, first_block_usage = 0,
    error_handler = 0x0}, mysql = 0x7fc94fa6d900, stmt_id = 3, flags = 0, state = MYSQL_STMT_USE_OR_STORE_CALLED, fields = 0x7fc94fa03250, field_count = 3,
  param_count = 0, send_types_to_server = 0 '\000', params = 0x0, bind = 0x7fc94fa034a0, result = {data = 0x7fc8a4f77bd8, embedded_info = 0x0, alloc = {
      free = 0x7fc94fb2ac00, used = 0x7fc81c100c00, pre_alloc = 0x7fc94fb2ac00, min_malloc = 32, block_size = 4072, block_num = 9, first_block_usage = 0,
      error_handler = 0x0}, rows = 3, fields = 0, extension = 0x0}, result_cursor = 0x7fc8a4f77bd8, bind_result_done = 0 '\000', bind_param_done = 1 '\001',
  upsert_status = {warning_count = 0, server_status = 2082, affected_rows = 3, last_insert_id = 0}, last_errno = 0, last_error = '\000' <repeats 512 times>,
  sqlstate = "00000", update_max_length = 1 '\001', prefetch_rows = 1, list = {prev = 0x0, next = 0x7fc94fa35320, data = 0x7fc94fa0e800},
  cursor_exists = 0 '\000', extension = 0x7fc94fa0b040, fetch_row_func = 0x56370a067afd <stmt_buffered_fetch>, execute_count = 1,
  default_rset_handler = 0x56370a068c63 <_mysql_stmt_use_result>, m = 0x0, array_size = 0, row_size = 0, prebind_params = 0, user_data = 0x0,
  result_callback = 0x0, param_callback = 0x0}
(gdb) p *MyRS->stmt->result.data
$4 = {next = 0x7fc895cd53d8, data = 0x7fc8a4f77bf0, length = 3474867094363060023}

Data on the 'stmt->result' appears to be invalid. Trying to reproduce the issue isolated in the test representing the POC (test_ps_async-t.cpp) revealed after replicating different operation conditions through the following query:

(SELECT id, k, REPEAT(c,2000) cc FROM test.sbtest1 LIMIT 10) 
UNION  (SELECT id, k, REPEAT(c,10) cc FROM test.sbtest1 LIMIT 10)

This query was trying to enforce a particular memory usage pattern, this revealed that there were several memory operations being performed over uninitialized values:

==1045151== 1 errors in context 1 of 9:
==1045151== Conditional jump or move depends on uninitialised value(s)
==1045151==    at 0x26450A: ma_alloc_root (ma_alloc.c:81)
==1045151==    by 0x265016: mthd_stmt_read_all_rows (mariadb_stmt.c:237)
==1045151==    by 0x26A637: mysql_stmt_store_result (mariadb_stmt.c:1824)
==1045151==    by 0x27DDC1: mysql_stmt_store_result_start_internal (mariadb_async.c:1469)
==1045151==    by 0x280022: my_context_spawn (ma_context.c:201)
==1045151==  Uninitialised value was created by a heap allocation
==1045151==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==1045151==    by 0x1B840F: ma_alloc_root(st_ma_mem_root*, unsigned long) (test_ps_async-t.cpp:77)
==1045151==    by 0x1B9A76: main (test_ps_async-t.cpp:382)

The root of this invalid operations should be fixed.

• ProxySQL version

v2.1.1

• OS version

NAME="Arch Linux"
PRETTY_NAME="Arch Linux"
ID=arch
BUILD_ID=rolling

• The steps to reproduce the issue

In the description above.