[PATCH] stop NULL pointer dereference in __alloc_pages

[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 ;-) ).