Fix: Chance to hit wasn't protected from becoming negative

Manual merge of pull request #3550. Credit to @newfrenchy83.

src/actions/attack.cpp

@@ -157,21 +157,21 @@ battle_context_unit_stats::battle_context_unit_stats(const unit& u,
 	}
 
 	// Compute chance to hit.
-	chance_to_hit = opp.defense_modifier(resources::gameboard->map().get_terrain(opp_loc)) + weapon->accuracy()
+	signed int cth = opp.defense_modifier(resources::gameboard->map().get_terrain(opp_loc)) + weapon->accuracy()
 		- (opp_weapon ? opp_weapon->parry() : 0);
 
-	if(chance_to_hit > 100) {
-		chance_to_hit = 100;
-	}
+	cth = utils::clamp(cth, 0, 100);
 
 	unit_ability_list cth_specials = weapon->get_specials("chance_to_hit");
-	unit_abilities::effect cth_effects(cth_specials, chance_to_hit, backstab_pos);
-	chance_to_hit = cth_effects.get_composite_value();
+	unit_abilities::effect cth_effects(cth_specials, cth, backstab_pos);
+	cth = cth_effects.get_composite_value();
 
 	if(opp.get_state("invulnerable")) {
-		chance_to_hit = 0;
+		cth = 0;
 	}
 
+	chance_to_hit = utils::clamp(cth, 0, 100);
+
 	// Compute base damage done with the weapon.
 	int base_damage = weapon->modified_damage(backstab_pos);
 
@@ -306,13 +306,13 @@ battle_context_unit_stats::battle_context_unit_stats(const unit_type* u_type,
 	}
 
 	signed int cth = 100 - opp_terrain_defense + weapon->accuracy() - (opp_weapon ? opp_weapon->parry() : 0);
-	cth = std::min(100, cth);
-	cth = std::max(0, cth);
-	chance_to_hit = cth;
+	cth = utils::clamp(cth, 0, 100);
 
 	unit_ability_list cth_specials = weapon->get_specials("chance_to_hit");
-	unit_abilities::effect cth_effects(cth_specials, chance_to_hit, backstab_pos);
-	chance_to_hit = cth_effects.get_composite_value();
+	unit_abilities::effect cth_effects(cth_specials, cth, backstab_pos);
+	cth = cth_effects.get_composite_value();
+
+	chance_to_hit = utils::clamp(cth, 0, 100);
 
 	int base_damage = weapon->modified_damage(backstab_pos);
 	int damage_multiplier = 100;