perf bench: Fix segfault at the end of an 'all' execution

At the end of

  $ perf bench all

the program segfaults because it attempts to dereference a NULL
pointer. Fix this fault.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1395964219-22173-4-git-send-email-artagnon@gmail.com
Signed-off-by: Jiri Olsa <jolsa@redhat.com>