Core/GameObjects: Fix crash with Summoning Ritual GOs

Fix a crash happening with GAMEOBJECT_TYPE_SUMMONING_RITUAL GameObjects if a Player clicked on the GO, logged out and let another Player click on it.
Issue added in 464c0c16116dfd22b3c104165ce6edd7b7f4a3d4
Fixes #11856
[TW278] - Core/Scripts: Prevent multiple spawns of Ulag

http://www.wowhead.com/quest=1819/ulag-the-cleaver