perf tests x86: Fix memory leak in sample_ustack()

The buf is not freed, when kernel failed to get stack map
and return.
Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Link: http://lkml.kernel.org/r/1398091024-7901-1-git-send-email-standby24x7@gmail.comSigned-off-by: Jiri Olsa <jolsa@redhat.com>

tools/perf/arch/x86/tests/dwarf-unwind.c

@@ -26,6 +26,7 @@ static int sample_ustack(struct perf_sample *sample,
 	map = map_groups__find(&thread->mg, MAP__FUNCTION, (u64) sp);
 	if (!map) {
 		pr_debug("failed to get stack map\n");
+		free(buf);
 		return -1;
 	}