sparc32 fix np dereference in do_unassigned_access

fix a potential null pointer dereference introduced in
commit	576c2cdc767ab9e2dc038fa4c99f22e53287a3de

Signed-off-by: Artyom Tarasenko <atar4qemu@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
