Core/Misc: Check GetVictim() for NULL before dereferencing it

Add additional NULL checks all around the code before dereferencing GetVictim() result.
Keep in mind UpdateVictim() result has nothing to do with GetVictim() result and the caller shouldn't assume anything about GetVictim() based on UpdateVictim().
Merge branch 'master' of github.com:TrinityCore/TrinityCore into 4.3.4

Conflicts:
	src/server/scripts/EasternKingdoms/ZulAman/boss_hexlord.cpp
	src/server/scripts/EasternKingdoms/ZulAman/boss_zuljin.cpp