Ensure that the number of added contacts never goes below 0.

This can happen when a REGISTER request is removing a contact.

(closes issue ASTERISK-21911)
Reported by: mdavenport


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@391902 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_sip_registrar.c

@@ -259,7 +259,7 @@ static pj_bool_t registrar_on_rx_request(struct pjsip_rx_data *rdata)
 		return PJ_TRUE;
 	}
 
-	if (((added - deleted) + (!aor->remove_existing ? ao2_container_count(contacts) : 0)) > aor->max_contacts) {
+	if ((MAX(added - deleted, 0) + (!aor->remove_existing ? ao2_container_count(contacts) : 0)) > aor->max_contacts) {
 		/* Enforce the maximum number of contacts */
 		pjsip_endpt_respond_stateless(ast_sip_get_pjsip_endpoint(), rdata, 403, NULL, NULL, NULL);
 		return PJ_TRUE;