Fix a segmentation fault when connecting to a server without a port

fix #3260

src/irc/irc_client.cpp

@@ -183,6 +183,11 @@ void IrcClient::start()
 {
   if (this->is_connecting() || this->is_connected())
     return;
+  if (this->ports_to_try.empty())
+    {
+      this->bridge.send_xmpp_message(this->hostname, "", "Can not connect to IRC server: no port specified.");
+      return;
+    }
   std::string port;
   bool tls;
   std::tie(port, tls) = this->ports_to_try.top();