gsm_04_08: check return code of osmo_mobile_identity_decode_from_l3()

The function gsm48_rx_cm_reest_req() is the only one where the return
code of osmo_mobile_identity_decode_from_l3() is not checked, lets check
it here too.

Change-Id: I37981205870b094b3a40a20197461208daa62698
Fixes: CID#211037
1 file changed