Fix seg fault when weapon is removed during attack

Events during attacks may modify either of the units involved. If, as a result, one of the units does not have a weapon of the type used any more, this previously caused a segmentation fault in unit_attack() in udisplay.cpp. The solution is to set the remaining number of strikes for the unit to zero. This prevents that unit from striking again, but lets the opponent continue its attack.

Fixes #4927 and #5914

src/actions/attack.cpp

@@ -1221,6 +1221,16 @@ bool attack::perform_hit(bool attacker_turn, statistics::attack_context& stats)
 	}
 
 	--attacker.n_attacks_;
+
+	// If an event removed a unit's weapon, set number of remaining attacks to zero
+	// for that unit, but let the other unit continue
+	if (attacker_stats->weapon == nullptr){
+        attacker.n_attacks_ = 0;
+	}
+	if (defender_stats->weapon == nullptr){
+        defender.n_attacks_ = 0;
+	}
+
 	return true;
 }