Fix single combat attacking target

[GregHib]

Because Target.attackable checks if the player.target is set to the target npc, it means any player can attack an npc even if it's currently in combat because character.target = target inside Combat.kts line 41 which occurs before attackable is checked.

This is because it's based on the assumption that target is only set when in combat. However this isn't true as target is set before entering combat and then removed if not attackable (self satisfying).

It also isn't a trivial fix as setting the target after checking attackable means the player can't re-attack the npc they're already fighting because target is set to null when combat stops for the split second when re-attacking.