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>

tools/perf/util/probe-event.c

@@ -1820,11 +1820,15 @@ static int convert_to_probe_trace_events(struct perf_probe_event *pev,
 		ret = -ENOMEM;
 		goto error;
 	}
-	tev->point.module = strdup(module);
-	if (tev->point.module == NULL) {
-		ret = -ENOMEM;
-		goto error;
+
+	if (module) {
+		tev->point.module = strdup(module);
+		if (tev->point.module == NULL) {
+			ret = -ENOMEM;
+			goto error;
+		}
 	}
+
 	tev->point.offset = pev->point.offset;
 	tev->point.retprobe = pev->point.retprobe;
 	tev->nargs = pev->nargs;