Use NULL to trigger unassigned memory error on 32-bit accesses instead
of assert(0) as suggested by Blue Swirl.
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5141 c046a42c-6fe2-441c-8c8c-71466251a162