ARM: fix segfault

Fix a bug in bccd9ec5f098668576342c83d90d6d6833d61d33,
target-arm/op_helper.c missed a change unlike all other targets.
This lead to a NULL pointer dereferences.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
