transaction: reject calls from unidentified subscribers

A valid subscriber is indespensible when allocating a new
transaction. Return NULL if no subscriber is supplied. This
will cause unidentified subscribers to be rejected.

Note: Under normal conditions, the problem does not occour,
but it is still possible that a misbehaving MS might trigger
the problem by sending a SETUP command before authenticating
the subscriber. (unencrypted networks)

Change-Id: Ia8739b6e329ab02c0064270d02ad1d6ee245520d

src/libmsc/transaction.c

@@ -88,6 +88,13 @@ struct gsm_trans *trans_alloc(struct gsm_network *net,
 
 	DEBUGP(DCC, "subscr=%p, net=%p\n", vsub, net);
 
+	/* a valid subscriber is indispensable */
+	if (vsub == NULL) {
+		LOGP(DCC, LOGL_NOTICE,
+		     "unable to alloc transaction, invalid subscriber (NULL)\n");
+		return NULL;
+	}
+
 	trans = talloc_zero(tall_trans_ctx, struct gsm_trans);
 	if (!trans)
 		return NULL;