powerpc: Fix mpic_resume on early G5 macs

mpic_resume() on G5 macs blindly dereferences mpic->fixups, but
it may legitimately be NULL (as on PowerMac7,2).  Add an explicit
check.

This fixes suspend-to-disk with one processor (maxcpus=1) for me.
Signed-off-by: Alastair Bridgewater <alastair.bridgewater@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/sysdev/mpic.c

@@ -1666,7 +1666,7 @@ static int mpic_resume(struct sys_device *dev)
 			       mpic->save_data[i].dest);
 
 #ifdef CONFIG_MPIC_U3_HT_IRQS
-	{
+	if (mpic->fixups) {
 		struct mpic_irq_fixup *fixup = &mpic->fixups[i];
 
 		if (fixup->base) {