fix crash when fake player dig slade (#46)

When a fake player, for example a node, dig slade, it return player ~= nil but it is not a current player, the server_diggable_only function tries to check the privs of the node, then this crashes

Here we use the built-in check player:is_player() so the node privs are not checked

This crash occurred in the technic mod when the corium eats the slade

df_underworld_items/slade.lua

@@ -3,7 +3,7 @@ local S = minetest.get_translator(minetest.get_current_modname())
 local invulnerable = df_underworld_items.config.invulnerable_slade and not minetest.settings:get_bool("creative_mode")
 
 local server_diggable_only = function(pos, player)
-	if player then
+	if player and player:is_player() then
 		return minetest.check_player_privs(player, "server")
 	end
 	return false
@@ -181,4 +181,4 @@ end
 
 if minetest.get_modpath("mesecons_mvps") and df_underworld_items.config.invulnerable_slade then
 	mesecon.register_mvps_stopper("df_underworld_items:slade")
-end
+end