RemotePlayer: make peer ID always reflect the validity of PlayerSAO (#14317)

Upon disconnect, RemotePlayer still had a peer ID assigned even though
the PlayerSAO object was maked as gone (for removal). This commit makes
that the following always holds true:

	(!sao || sao->isGone()) === (peer_id == PEER_ID_INEXISTENT)

src/remoteplayer.cpp

@@ -69,6 +69,11 @@ RemotePlayer::RemotePlayer(const char *name, IItemDefManager *idef):
 	m_star_params   = SkyboxDefaults::getStarDefaults();
 }
 
+RemotePlayer::~RemotePlayer()
+{
+	if (m_sao)
+		m_sao->setPlayer(nullptr);
+}
 
 RemotePlayerChatResult RemotePlayer::canSendChatMessage()
 {