perf events: Fix false positive build warning with older GCC's

gcc 4.2.1 produces:

 util/probe-event.c: In function 'add_perf_probe_events':
 util/probe-event.c:883: warning: 'tev' may be used uninitialized in this function
 make: *** [util/probe-event.o] Error 1

Newer GCCs get this right.

To work it around, initialize the variable to NULL so that older GCCs see
it as initialized too.

Cc: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100316220612.32050.33806.stgit@localhost6.localdomain6>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

tools/perf/util/probe-event.c

@@ -880,7 +880,7 @@ static void __add_kprobe_trace_events(struct perf_probe_event *pev,
 				      int ntevs, bool allow_suffix)
 {
 	int i, fd;
-	struct kprobe_trace_event *tev;
+	struct kprobe_trace_event *tev = NULL;
 	char buf[64];
 	const char *event, *group;
 	struct strlist *namelist;