do not edit the zone currently iterated on (#4345)

this can cause the iterator to become invalidated which will crash but
because of the data not always being moved it will often still work as
intended, giving the idea that it is random

common/server_game.cpp

@@ -577,19 +577,17 @@ void Server_Game::unattachCards(GameEventStorage &ges, Server_Player *player)
 
     for (auto zone : player->getZones()) {
         for (auto card : zone->getCards()) {
-            if (card == nullptr) {
-                continue;
-            }
-
-            const auto &attachedCardsBase = card->getAttachedCards();
-            if (attachedCardsBase.isEmpty()) {
-                continue;
-            }
-
             // Make a copy of the list because the original one gets modified during the loop
-            QList<Server_Card *> attachedCards = {attachedCardsBase};
+            QList<Server_Card *> attachedCards = card->getAttachedCards();
             for (Server_Card *attachedCard : attachedCards) {
-                attachedCard->getZone()->getPlayer()->unattachCard(ges, attachedCard);
+                auto otherPlayer = attachedCard->getZone()->getPlayer();
+                // do not modify the current player's zone!
+                // this would cause the current card iterator to be invalidated!
+                // we only have to return cards owned by other players
+                // because the current player is leaving the game anyway
+                if (otherPlayer != player) {
+                    otherPlayer->unattachCard(ges, attachedCard);
+                }
             }
         }
     }