perf evlist: Fix for double free in tools/perf stat

Fix for double free bug in tools/perf due to dangling thread_map pointer
in perf_evlist struct.

Code path excercised when perf stat -C switch is used but not set and is
followed by another switch.

Example:

  perf stat -C -e.

Signed-off-by: Yasser Shalabi <yassershalabi@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1412437077-13109-1-git-send-email-yassershalabi@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evlist.c

@@ -1003,6 +1003,7 @@ int perf_evlist__create_maps(struct perf_evlist *evlist, struct target *target)
 
 out_delete_threads:
 	thread_map__delete(evlist->threads);
+	evlist->threads = NULL;
 	return -1;
 }