Prevent kart placeholders from null pointer dereferencing when setting karts

Probably, not sure, it is almost certainly the case but I might be mistaken

Author: kimden

src/network/protocols/server_lobby.cpp

@@ -1982,6 +1982,8 @@ void ServerLobby::asynchronousUpdate()
             for (unsigned i = 0; i < players.size(); i++)
             {
                 std::string current_kart = players[i]->getKartName();
+                if (!players[i]->getPeer().get())
+                    continue;
                 if (areKartFiltersIgnoringKarts())
                     current_kart = "";
                 std::string name = StringUtils::wideToUtf8(players[i]->getName());