xen/events: BUG() when we can't allocate our event->irq array.

In case we can't allocate we are doomed. We should BUG_ON
instead of trying to dereference it later on.
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[v1: Use BUG_ON instead of BUG]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/events.c

@@ -1670,6 +1670,7 @@ void __init xen_init_IRQ(void)
 
 	evtchn_to_irq = kcalloc(NR_EVENT_CHANNELS, sizeof(*evtchn_to_irq),
 				    GFP_KERNEL);
+	BUG_ON(!evtchn_to_irq);
 	for (i = 0; i < NR_EVENT_CHANNELS; i++)
 		evtchn_to_irq[i] = -1;