perf: Fix free_event()

With the context rework stuff we can actually end up freeing an event
before it gets attached to a context.

Reported-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_event.c

@@ -2150,7 +2150,9 @@ static void free_event(struct perf_event *event)
 	if (event->destroy)
 		event->destroy(event);
 
-	put_ctx(event->ctx);
+	if (event->ctx)
+		put_ctx(event->ctx);
+
 	call_rcu(&event->rcu_head, free_event_rcu);
 }