Fix bug #25538: Monte Carlo damage prediction relies on UB

IEEE 754 specifies that dividing a positive number with zero yields
positive infinity, but the C++ language doesn't guarantee IEEE 754
semantics. As a result, UBSan complains about floating-point division by
zero. Wesnoth requires IEEE 754 anyway, but it's still good to get rid of
the UBSan warning.

The code now avoids the division if the divisor is zero.

src/attack_prediction.cpp

@@ -1278,10 +1278,10 @@ monte_carlo_combat_matrix::monte_carlo_combat_matrix(unsigned int a_max_hp, unsi
 	a_split_(a_split), b_split_(b_split), rounds_(rounds), a_hit_chance_(a_hit_chance), b_hit_chance_(b_hit_chance),
 	a_initially_slowed_chance_(a_initially_slowed_chance), b_initially_slowed_chance_(b_initially_slowed_chance)
 {
-	scale_probabilities(a_summary[0], a_initial_, 1.0 / (1.0 - a_initially_slowed_chance), a_hp);
-	scale_probabilities(a_summary[1], a_initial_slowed_, 1.0 / a_initially_slowed_chance, a_hp);
-	scale_probabilities(b_summary[0], b_initial_, 1.0 / (1.0 - b_initially_slowed_chance), b_hp);
-	scale_probabilities(b_summary[1], b_initial_slowed_, 1.0 / b_initially_slowed_chance, b_hp);
+	scale_probabilities(a_summary[0], a_initial_, 1.0 - a_initially_slowed_chance, a_hp);
+	scale_probabilities(a_summary[1], a_initial_slowed_, a_initially_slowed_chance, a_hp);
+	scale_probabilities(b_summary[0], b_initial_, 1.0 - b_initially_slowed_chance, b_hp);
+	scale_probabilities(b_summary[1], b_initial_slowed_, b_initially_slowed_chance, b_hp);
 
 	clear();
 }
@@ -1435,9 +1435,9 @@ unsigned int monte_carlo_combat_matrix::calc_blows_b(unsigned int b_hp) const
 	return it->strikes;
 }
 
-void monte_carlo_combat_matrix::scale_probabilities(const std::vector<double>& source, std::vector<double>& target, double multiplier, unsigned int singular_hp)
+void monte_carlo_combat_matrix::scale_probabilities(const std::vector<double>& source, std::vector<double>& target, double divisor, unsigned int singular_hp)
 {
-	if (std::isinf(multiplier))
+	if (divisor == 0.0)
 	{
 		// Happens if the "target" HP distribution vector isn't used,
 		// in which case it's not necessary to scale the probabilities.
@@ -1451,7 +1451,7 @@ void monte_carlo_combat_matrix::scale_probabilities(const std::vector<double>& s
 	}
 	else
 	{
-		std::transform(source.begin(), source.end(), std::back_inserter(target), [=](double prob){ return multiplier * prob; });
+		std::transform(source.begin(), source.end(), std::back_inserter(target), [=](double prob){ return prob / divisor; });
 	}
 
 	assert(std::abs(std::accumulate(target.begin(), target.end(), 0.0) - 1.0) < 0.001);