Add checks to enchanting table formspec/inventory to prevent crashing in some situations.

enchanting.lua

@@ -59,7 +59,9 @@ function enchanting.on_put(pos, listname, _, stack)
 end
 
 function enchanting.fields(pos, _, fields, sender)
-	if fields.quit then return end
+	if not next(fields) or fields.quit then
+		return
+	end
 	local inv = minetest.get_meta(pos):get_inventory()
 	local tool = inv:get_stack("tool", 1)
 	local mese = inv:get_stack("mese", 1)
@@ -83,6 +85,9 @@ function enchanting.dig(pos)
 end
 
 local function allowed(tool)
+	if not tool then
+		return false
+	end
 	for item in pairs(minetest.registered_tools) do
 		if item:find("enchanted_"..tool) then return true end
 	end