Fix #2117: crash if either combatant was guaranteed to die

Regression from commit f6c4f3d.

The code divided by zero and the probability to stay unscathed ended up as
NaN, which triggered an assertion failure if the AI simulated one more
fight for either combatant.

src/attack_prediction.cpp

@@ -2097,7 +2097,8 @@ void complex_fight(attack_prediction_mode mode,
 					double first_hit = hit_chance * opp_hit_unknown;
 					opp_hit += first_hit;
 					opp_hit_unknown -= first_hit;
-					double this_hit_killed_b = (pm->dead_prob_b() - b_already_dead) / ((1.0 - b_already_dead) * (1.0 - pm->dead_prob_a()));
+					double both_were_alive = (1.0 - b_already_dead) * (1.0 - pm->dead_prob_a());
+					double this_hit_killed_b = both_were_alive != 0.0 ? (pm->dead_prob_b() - b_already_dead) / both_were_alive : 1.0;
 					self_hit_unknown *= (1.0 - this_hit_killed_b);
 				}
 				if(i < opp_strikes) {
@@ -2109,7 +2110,8 @@ void complex_fight(attack_prediction_mode mode,
 					double first_hit = opp_hit_chance * self_hit_unknown;
 					self_hit += first_hit;
 					self_hit_unknown -= first_hit;
-					double this_hit_killed_a = (pm->dead_prob_a() - a_already_dead) / ((1.0 - a_already_dead) * (1.0 - pm->dead_prob_b()));
+					double both_were_alive = (1.0 - a_already_dead) * (1.0 - pm->dead_prob_b());
+					double this_hit_killed_a = both_were_alive != 0.0 ? (pm->dead_prob_a() - a_already_dead) / both_were_alive : 1.0;
 					opp_hit_unknown *= (1.0 - this_hit_killed_a);
 				}
 			}