fix: send SNDCP XID only on GERAN Gb contexts

Add a condition for GERAN Gb.

SNDCP and IuPS were developed on separate branches, and the merge results in
code trying to use an llme on a UTRAN Iu context where the llme is NULL,
leading to stack corruption upon PDP ctx act.

Change-Id: Ibb20d738c1b64d36630ce8eceb54c33ba4f1b003
diff --git a/openbsc/src/gprs/sgsn_libgtp.c b/openbsc/src/gprs/sgsn_libgtp.c
index 127ee30..29c9f06 100644
--- a/openbsc/src/gprs/sgsn_libgtp.c
+++ b/openbsc/src/gprs/sgsn_libgtp.c
@@ -331,11 +331,13 @@
 	if (rc < 0)
 		return rc;
 
-	/* Send SNDCP XID to MS */
-	lle = &pctx->mm->gb.llme->lle[pctx->sapi];
-	rc = sndcp_sn_xid_req(lle,pctx->nsapi);
-	if (rc < 0)
-		return rc;
+	if (pctx->mm->ran_type == MM_CTX_T_GERAN_Gb) {
+		/* Send SNDCP XID to MS */
+		lle = &pctx->mm->gb.llme->lle[pctx->sapi];
+		rc = sndcp_sn_xid_req(lle,pctx->nsapi);
+		if (rc < 0)
+			return rc;
+	}
 
 	return 0;
 }