This fixes a crash that occurs in Mystic Realm's final boss, as a result of it calling
A_SplitShot, which in turns calls
A_FaceTarget from within, and the actor has no target.
(It seems that despite
A_FaceTarget checking if actor target exists, the ASM of this code within
A_SplitShot doesn't include this check for some reason (???). Anyway, I added an additional check in
A_SplitShot itself so it shouldn't be doing anything at all now when no target exists)