fix nullpointer: in gsm48_rx_gmm_ra_upd_req()
This caused frequent crashes at 36c3. The "proper" fix is probably elsewhere
(lynxis mentions an unfinished patch), but at least this prevented some crashes
during active operation.
Once this is merged, we can (re)enable SGSN_Tests_Iu.TC_geran_attach_iu_rau,
which tests exactly for this scenario: A Subscriber / MM context that is so
far attached via GERAN, but now receives a RAU via UTRAN/Iu.
Closes: OS#4339
Change-Id: Ifde15dc4151d84748f0e67b32c9c260cb2d9d8fc