x86: Fix NULL pointer access in irq_force_complete_move() for Xen guests

Upstream PV guests fail to boot because of a NULL pointer in
irq_force_complete_move().  It is possible that xen guests have
irq_desc->chip_data = NULL.

Test for NULL chip_data pointer before attempting to complete an irq move.
Signed-off-by: Prarit Bhargava <prarit@redhat.com>
LKML-Reference: <20100427152434.16193.49104.sendpatchset@prarit.bos.redhat.com>
Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: <stable@kernel.org> [2.6.33]

arch/x86/kernel/apic/io_apic.c

@@ -2545,6 +2545,9 @@ void irq_force_complete_move(int irq)
 	struct irq_desc *desc = irq_to_desc(irq);
 	struct irq_cfg *cfg = desc->chip_data;
 
+	if (!cfg)
+		return;
+
 	__irq_complete_move(&desc, cfg->vector);
 }
 #else