Core: Fix wrong cast in CanHaveThreatList.

All Pets are Minion. All Minions are NOT Pets.
All Pets are Guardian. All Guardians are NOT Pets.

Wrong cast can make random crashes.
This should fix all these impossible crashes we get from time to time.

NB: as Guardians and Pets are Minions, we don't need the two last verifications.
But asserts are sometimes useful.
original patch by Arktus

Signed-off-by: bootz <bootz@projectskyfire.org>