Actor::GetCombatDetails: added a research todo

it's a bit odd that goblins hit that part, so perhaps it should be limited
Actor::PerformAttack: never produce negative damage from the main roll

10gob in iwd2 start has ingrained -3 luck, boring stats and a 1d4 weapon,
so they kept hitting pcs with negative damage, healing them :)

Did not see it sooner, as the main install had the armour rebalancing mod,
which replaces ac for damage soaking