Don't go out of bounds when picking a race for a mission

src/Geoscape/GeoscapeState.cpp

@@ -2643,6 +2643,11 @@ bool GeoscapeState::processCommand(RuleMissionScript *command)
 		missionRace = command->generate(month, GEN_RACE);
 	}
 
+	if (missionRace.empty())
+	{
+		throw Exception("Error proccessing mission script named: " + command->getType() + ", mission type: " + missionType + " has no available races");
+	}
+
 	// we're bound to end up with typos, so let's throw an exception instead of simply returning false
 	// that way, the modder can fix their mistake
 	if (mod->getAlienRace(missionRace) == 0)

src/Mod/RuleAlienMission.cpp

@@ -142,9 +142,10 @@ void RuleAlienMission::load(const YAML::Node &node)
  */
 std::string RuleAlienMission::generateRace(const size_t monthsPassed) const
 {
-	std::vector<std::pair<size_t, WeightedOptions*> >::const_reverse_iterator rc = _raceDistribution.rbegin();
-	while (monthsPassed < rc->first)
-		++rc;
+	std::vector<std::pair<size_t, WeightedOptions*> >::const_reverse_iterator rc;
+	for (rc = _raceDistribution.rbegin(); rc != _raceDistribution.rend() && monthsPassed < rc->first; ++rc);
+	if (rc == _raceDistribution.rend())
+		return "";
 	return rc->second->choose();
 }