xen: fix memory access violation bug when CONFIG_HAVE_SPARSE_IRQ is enabled

When sparse IRQs are enabled, it is not safe to assume an IRQ descriptor
exists for every possible IRQ.  This patch causes init_evtchn_cpu_bindings
to skip initialisation of IRQ descriptors which don't exist.
Signed-off-by: Alex Nixon <alex.nixon@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

drivers/xen/events.c

@@ -141,6 +141,8 @@ static void init_evtchn_cpu_bindings(void)
 	/* By default all event channels notify CPU#0. */
 	for (i = 0; i < nr_irqs; i++) {
 		struct irq_desc *desc = irq_to_desc(i);
+		if (!desc)
+			continue;
 		desc->affinity = cpumask_of_cpu(0);
 	}
 #endif