perf machine: Guard against NULL in machine__exit()

commit 4a2233b upstream.

A recent fix for 'perf trace' introduced a bug where
machine__exit(trace->host) could be called while trace->host was still
NULL, so make this more robust by guarding against NULL, just like
free() does.

The problem happens, for instance, when !root users try to run 'perf
trace':

  [acme@jouet linux]$ trace
  Error:	No permissions to read /sys/kernel/debug/tracing/events/raw_syscalls/sys_(enter|exit)
  Hint:	Try 'sudo mount -o remount,mode=755 /sys/kernel/debug/tracing'

  perf: Segmentation fault
  Obtained 7 stack frames.
  [0x4f1b2e]
  /lib64/libc.so.6(+0x3671f) [0x7f43a1dd971f]
  [0x4f3fec]
  [0x47468b]
  [0x42a2db]
  /lib64/libc.so.6(__libc_start_main+0xe9) [0x7f43a1dc3509]
  [0x42a6c9]
  Segmentation fault (core dumped)
  [acme@jouet linux]$

Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Andrei Vagin <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Vasily Averin <[email protected]>
Cc: Wang Nan <[email protected]>
Fixes: 33974a4 ("perf trace: Call machine__exit() at exit")
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>
Signed-off-by: Tommi Rantala <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

tools/perf/util/machine.c

@@ -156,6 +156,9 @@ void machine__delete_threads(struct machine *machine)
 
 void machine__exit(struct machine *machine)
 {
+	if (machine == NULL)
+		return;
+
 	machine__destroy_kernel_maps(machine);
 	map_groups__exit(&machine->kmaps);
 	dsos__exit(&machine->dsos);