Server: Fix Server::Send exception caused by leaving players

Leaving players will have PEER_ID_INVALID for database saving purposes.
This patch clarifies that, and properly protects the batch send function.

Bug introduced by 5f5ea132.

src/server/player_sao.cpp

@@ -50,6 +50,8 @@ PlayerSAO::PlayerSAO(ServerEnvironment *env_, RemotePlayer *player_, session_t p
 		m_armor_groups["immortal"] = 1;
 }
 
+// PlayerSAO::~PlayerSAO(): eventually deleted by `ActiveObjectMgr::removeObject`
+
 void PlayerSAO::finalize(RemotePlayer *player, const std::set<std::string> &privs)
 {
 	assert(player);