gsm0408_rcv_cc: guard against NULL subscriber

Check conn->subscr against NULL.

gsm0408_rcv_cc() dereferences many conn members without checking presence: the
bts and lchan members may be expected to be NULL in the ongoing MSC split and
3G developments.

But the conn->subscr is initially NULL, so an MS sending a CC message before
something like a LU or CM Service Request will result in a segfault. Prevent
that.

Note: the upcoming VLR will be more restrictive on what messages are processed,
this is a "backport" to the situation on current master.

Change-Id: If067db7cc0dd3210d9eb1da15be6b637795a3ecf

openbsc/src/libmsc/gsm_04_08.c

@@ -3605,6 +3605,11 @@ static int gsm0408_rcv_cc(struct gsm_subscriber_connection *conn, struct msgb *m
 		return -EINVAL;
 	}
 
+	if (!conn->subscr) {
+		LOGP(DCC, LOGL_ERROR, "Invalid conn, no subscriber\n");
+		return -EINVAL;
+	}
+
 	/* Find transaction */
 	trans = trans_find_by_id(conn, GSM48_PDISC_CC, transaction_id);