check player valid when respawn, cos update healt cannot check

* update_health canot made a check player cos its called already
  in other places where such check is made, but at respawn moment
  the player object is not checked so we must made it manually
  and not a double check in update_healt due performancde issues

init.lua

@@ -594,7 +594,9 @@ minetest.register_on_player_hpchange(function(player)
 end)
 
 minetest.register_on_respawnplayer(function(player)
+	if player_exists(player) then
 	update_health(player)
+	end
 	hb.hide_hudbar(player, "breath")
 end)