[SGSN] Fix segfault when passing re-assembled SN-PDU to GMM

sgsn_rx_sndcp_ud_ind() can no longer make the assumption that msgb_bcid() is
valid, as this is only true for an un-fragmented SN-PDU.  So instead,
we now store the RAID in the SNDCP Entity and pass it as an explicit
argument to sgsn_rx_sndcp_ud_ind().
diff --git a/openbsc/src/gprs/gprs_sndcp.c b/openbsc/src/gprs/gprs_sndcp.c
index b352aeb..6e2d314 100644
--- a/openbsc/src/gprs/gprs_sndcp.c
+++ b/openbsc/src/gprs/gprs_sndcp.c
@@ -170,8 +170,8 @@
 
 	/* actually send the N-PDU to the SGSN core code, which then
 	 * hands it off to the correct GTP tunnel + GGSN via gtp_data_req() */
-	return sgsn_rx_sndcp_ud_ind(sne->lle->llme->tlli, sne->nsapi, msg,
-				    sne->defrag.tot_len, npdu);
+	return sgsn_rx_sndcp_ud_ind(&sne->ra_id, sne->lle->llme->tlli,
+				    sne->nsapi, msg, sne->defrag.tot_len, npdu);
 }
 
 static int defrag_input(struct gprs_sndcp_entity *sne, struct msgb *msg, uint8_t *hdr)
@@ -517,6 +517,8 @@
 			lle->llme->tlli, lle->sapi, sch->nsapi);
 		return -EIO;
 	}
+	/* FIXME: move this RA_ID up to the LLME or even higher */
+	bssgp_parse_cell_id(&sne->ra_id, msgb_bcid(msg));
 
 	if (!sch->first || sch->more) {
 #if 0
@@ -542,7 +544,7 @@
 	}
 	/* actually send the N-PDU to the SGSN core code, which then
 	 * hands it off to the correct GTP tunnel + GGSN via gtp_data_req() */
-	return sgsn_rx_sndcp_ud_ind(lle->llme->tlli, sne->nsapi, msg, npdu_len, npdu);
+	return sgsn_rx_sndcp_ud_ind(&sne->ra_id, lle->llme->tlli, sne->nsapi, msg, npdu_len, npdu);
 }
 
 /* Section 5.1.2.1 LL-RESET.ind */