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.com


Signed-off-by: Jiri Olsa <jolsa@redhat.com>