When hosting, don't try to connect if listening failed.

If another instance of the server is running on the same computer, this
would cause Dolphin to confusingly connect to it.

Source/Core/DolphinWX/Src/NetWindow.cpp

@@ -254,9 +254,12 @@ void NetPlaySetupDiag::OnHost(wxCommandEvent&)
 		if(m_upnp_chk->GetValue())
 			netplay_server->TryPortmapping(port);
 #endif
+		MakeNetPlayDiag(port, game, true);
+	}
+	else
+	{
+		PanicAlertT("Failed to listen.  Is another instance of the NetPlay server running?");
 	}
-
-	MakeNetPlayDiag(port, game, true);
 }
 
 void NetPlaySetupDiag::OnJoin(wxCommandEvent&)