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

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 <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jiri Olsa <[email protected]>