[POWERPC] Fix warning in prom_parse.c of_irq_map_oldworld()

This function spews a warning due to possible use of an uninitialized
variable. This can happen on broken device-trees or when called with
a NULL argument. Makes ure we properly fail instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/kernel/prom_parse.c b/arch/powerpc/kernel/prom_parse.c
index ea6fd55..91b443c 100644
--- a/arch/powerpc/kernel/prom_parse.c
+++ b/arch/powerpc/kernel/prom_parse.c
@@ -916,7 +916,7 @@
 static int of_irq_map_oldworld(struct device_node *device, int index,
 			       struct of_irq *out_irq)
 {
-	const u32 *ints;
+	const u32 *ints = NULL;
 	int intlen;
 
 	/*