Blackfin arch: fix bug - jump_to_zero test case failed on noMPU kernel

The nompu code is now derived from the mpu code, and had the same problem -
no null pointer detection on ICPLBs.

Signed-off-by: Bernd Schmidt <bernds_cb1@t-online.de>
Cc: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>