[POWERPC] Fix crash in init_ipic_sysfs on efika

The global primary_ipic in arch/powerpc/sysdev/ipic.c can remain NULL
if ipic_init() fails, which will happen on machines that don't have an
ipic interrupt controller.  init_ipic_sysfs() will crash in that case.
Acked-by: NGrant Likely <grant.likely@secretlab.ca>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/sysdev/ipic.c

@@ -906,7 +906,7 @@ static int __init init_ipic_sysfs(void)
 {
 	int rc;
 
-	if (!primary_ipic->regs)
+	if (!primary_ipic || !primary_ipic->regs)
 		return -ENODEV;
 	printk(KERN_DEBUG "Registering ipic with sysfs...\n");