Only call server request callback if not NULL.

dll/steam_matchmaking_servers.cpp

@@ -424,6 +424,7 @@ void Steam_Matchmaking_Servers::RunCallbacks()
         if (r.cancelled || r.completed) continue;
         int i = 0;
 
+        if (r.callbacks) {
             for (auto &g : r.gameservers_filtered) {
                 PRINT_DEBUG("REQUESTS server responded cb %p\n", r.id);
                 r.callbacks->ServerResponded(r.id, i);
@@ -436,6 +437,7 @@ void Steam_Matchmaking_Servers::RunCallbacks()
                 r.callbacks->RefreshComplete(r.id, eNoServersListedOnMasterServer);
             }
         }
+    }
 
     std::vector <struct Steam_Matchmaking_Servers_Direct_IP_Request> direct_ip_requests_temp = direct_ip_requests;
     direct_ip_requests.clear();