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/remoteplayer.h

@@ -123,7 +123,8 @@ public:
 
 	bool inventory_formspec_overridden = false;
 
-	/// returns PEER_ID_INEXISTENT when PlayerSAO is not ready
+	/// returns PEER_ID_INEXISTENT when PlayerSAO is either
+	/// not yet present or about to be removed (e.g. leave).
 	session_t getPeerId() const { return m_peer_id; }
 
 	void setPeerId(session_t peer_id) { m_peer_id = peer_id; }

src/server.cpp

@@ -1618,7 +1618,7 @@ void Server::SendSpawnParticles(RemotePlayer *player,
 	const float radius_sq = radius * radius;
 
 	PlayerSAO *sao = player->getPlayerSAO();
-	if (!sao)
+	if (!sao || sao->isGone())
 		return;
 
 	std::ostringstream particle_batch_data(std::ios_base::binary);

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);