Fix [damage] weapon special produces unexpected damage values

Fix wesnoth#9467

src/units/abilities.cpp

@@ -2474,7 +2474,9 @@ effect::effect(const unit_ability_list& list, int def, const_attack_ptr att, EFF
 		effect_list_.push_back(val.second);
 	}
 
-	composite_value_ = static_cast<int>((value_set + addition + substraction) * multiplier / divisor);
+	double temp_value = (value_set + addition + substraction) * multiplier / divisor;
+	composite_value_ = static_cast<int>(temp_value);
+	composite_value_ = ((temp_value - composite_value_) >= 0.5) ? std::round(temp_value) : composite_value_;
 	//clamp what if min_value < max_value or one attribute only used.
 	if(max_value && min_value && *min_value < *max_value) {
 		composite_value_ = std::clamp(*min_value, *max_value, composite_value_);