Prevent iterator invalidation when cleaning up summons

apps/openmw/mwgui/container.cpp

@@ -277,8 +277,9 @@ namespace MWGui
                             auto it = std::find_if(summons.begin(), summons.end(), [&] (const auto& entry) { return entry.second == creatureStats.getActorId(); });
                             if(it != summons.end())
                             {
-                                MWMechanics::purgeSummonEffect(summoner, *it);
+                                auto summon = *it;
                                 summons.erase(it);
+                                MWMechanics::purgeSummonEffect(summoner, summon);
                                 break;
                             }
                         }

apps/openmw/mwmechanics/summoning.cpp

@@ -125,8 +125,9 @@ namespace MWMechanics
             if (!ptr.isEmpty() && ptr.getClass().getCreatureStats(ptr).isDead() && ptr.getClass().getCreatureStats(ptr).isDeathAnimationFinished())
             {
                 // Purge the magic effect so a new creature can be summoned if desired
-                purgeSummonEffect(summoner, *it);
+                auto summon = *it;
                 creatureMap.erase(it++);
+                purgeSummonEffect(summoner, summon);
             }
             else
                 ++it;