Scripts/Misc: Ensure Creatures are in instances when required

Create ScriptedAIs that require a InstanceScript reference only if the InstanceScript exists, so if these Creatures are in an instance. ScriptedAIs that don't require a InstanceScript reference have not been modified.
This fixes many possible NULL dereference crashes happening when spawning a scripted Creature outside of an instance.
Fixed a GetOwner() and a ToPlayer() NULL dereference crashes too.