Pet: Tryfix double unsummon crash due to visibility

Close cmangos/mangos-wotlk#508

Credit @insunaa for the general idea

src/game/Entities/Pet.cpp

@@ -2294,7 +2294,17 @@ void Pet::ForcedDespawn(uint32 timeMSToDespawn, bool onlyAlive)
     if (IsAlive())
         SetDeathState(JUST_DIED);
 
-    RemoveCorpse(true);                                     // force corpse removal in the same grid
+    if (GetDeathState() == CORPSE) // rest of despawn cleanup meant to be done by Unsummon
+    {
+        if (AI())
+        {
+            uint32 respawnDelay = 0;
+            AI()->CorpseRemoved(respawnDelay);
+        }
+
+        if (InstanceData* mapInstance = GetInstanceData())
+            mapInstance->OnCreatureDespawn(this);
+    }
 
     Unsummon(PET_SAVE_NOT_IN_SLOT, owner);
 }