Core/Chat: Fix assert triggered by .npc add

Fix an assert in Aura::_UnapplyForTarget() caused by ".npc add" changing the guid of the spawned Creature.

src/server/scripts/Commands/cs_npc.cpp

@@ -275,7 +275,11 @@ public:
 
         uint32 db_guid = creature->GetDBTableGUIDLow();
 
-        // To call _LoadGoods(); _LoadQuests(); CreateTrainerSpells();
+        // To call _LoadGoods(); _LoadQuests(); CreateTrainerSpells()
+        // current "creature" variable is deleted and created fresh new, otherwise old values might trigger asserts or cause undefined behavior
+        creature->CleanupsBeforeDelete();
+        delete creature;
+        creature = new Creature();
         if (!creature->LoadCreatureFromDB(db_guid, map))
         {
             delete creature;