perf probe: Fix coredump introduced by probe module option

perf will coredump if the user doesn't give the "-m" option in probe
command, this patch fixes it.

[root@localhost perf]# ./perf probe --add='PROBE'
Segmentation fault (core dumped)

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1311602888-2389-1-git-send-email-bookjovi@gmail.com
Signed-off-by: Jovi Zhang <bookjovi@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>