Fix crash on attacking town with garrison but without defending heroes

lib/mapObjects/CGTownInstance.cpp

@@ -310,7 +310,7 @@ void CGTownInstance::onHeroVisit(IGameEventCallback & gameEvents, const CGHeroIn
 			const CArmedInstance * defendingArmy = defendingHero ? static_cast<const CArmedInstance*>(defendingHero) : this;
 			const bool isBattleOutside = isBattleOutsideTown(defendingHero);
 
-			if(!isBattleOutside && defendingHero == getVisitingHero())
+			if(!isBattleOutside && defendingHero && defendingHero == getVisitingHero())
 				mergeGarrisonOnSiege(gameEvents);
 
 			gameEvents.startBattle(h, defendingArmy, getSightCenter(), h, defendingHero, BattleLayout::createDefaultLayout(*cb, h, defendingArmy), (isBattleOutside ? nullptr : this));