fixed erroneus handling of many players with no peer existing at same time

src/environment.cpp

@@ -194,6 +194,7 @@ void Environment::addPlayer(Player *player)
 #ifndef SERVER
 	assert(player->isLocal() == false || getLocalPlayer() == NULL);
 #endif
+	if(player->peer_id != 0)
 		assert(getPlayer(player->peer_id) == NULL);
 	m_players.push_back(player);
 }

src/environment.h

@@ -73,6 +73,8 @@ public:
 
 private:
 	Map *m_map;
+	// peer_ids in here should be unique, except that there may be
+	// many 0s
 	core::list<Player*> m_players;
 	// Debug output goes here
 	std::ostream &m_dout;