[PATCH] stop NULL pointer dereference in __alloc_pages

This trivial patch will apply to both 2.4.19-pre7 and 2.5.8 with just line
offsets. It stops us from following a NULL pointer in classzone in the case
where there is a pgdat without a fully populated zone list (ie a node with
no ZONE_NORMAL on an ia32 NUMA machine). Without this patch, ia32
NUMA machines won't even boot - we dereference the classzone ptr
a few lines further down (or try to ;-) ).

mm/page_alloc.c

@@ -340,6 +340,8 @@ struct page * __alloc_pages(unsigned int gfp_mask, unsigned int order, zonelist_
 
 	zone = zonelist->zones;
 	classzone = *zone;
+	if (classzone == NULL)
+		return NULL;
 	min = 1UL << order;
 	for (;;) {
 		zone_t *z = *(zone++);