mm: fix null pointer dereference in wait_iff_congested()

An unintended consequence of commit 4ae0a48b

 ("mm: modify
pgdat_balanced() so that it also handles order-0") is that
wait_iff_congested() can now be called with NULL 'struct zone *'
producing kernel oops like this:

  BUG: unable to handle kernel NULL pointer dereference
  IP: [<ffffffff811542d9>] wait_iff_congested+0x59/0x140

This trivial patch fixes it.

Reported-by: Zhouping Liu <zliu@redhat.com>
Reported-and-tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Zlatko Calusic <zlatko.calusic@iskon.hr>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>