in gprs_gmm.c:gsm48_rx_gmm_ra_upd_req the variable reject_cause
is not initalized, which is ok, since it gets initalized before
the jump into the "rejected" path. However, the compiler still
throws a warning. This commit fixes the problem by preinitalizing
the reject_cause to GMM_CAUSE_PROTO_ERR_UNSPEC
Change-Id: I84cffb631e4cad3d4748512b47e3876208f53727