perf: Fix perf_event_init_context()

perf_pin_task_context() can return NULL but perf_event_init_context()
assumes it will not, correct this.
Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Link: http://lkml.kernel.org/r/20140505171428.GU26782@laptop.programming.kicks-ass.netSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/core.c

@@ -7729,6 +7729,8 @@ int perf_event_init_context(struct task_struct *child, int ctxn)
 	 * swapped under us.
 	 */
 	parent_ctx = perf_pin_task_context(parent, ctxn);
+	if (!parent_ctx)
+		return 0;
 
 	/*
 	 * No need to check if parent_ctx != NULL here; since we saw