Fix chest formspec protection (#540)

This closes #539.

technic_chests/register.lua

@@ -159,6 +159,16 @@ local function get_receive_fields(name, data)
 	return function(pos, formname, fields, sender)
 		local meta = minetest.get_meta(pos)
 		local page = "main"
+
+		local owner = meta:get_string("owner")
+		if owner ~= "" then
+			-- prevent modification of locked chests
+			if owner ~= sender:get_player_name() then return end
+		elseif not fields.quit then
+			-- prevent modification of protected chests
+			if minetest.is_protected(pos, sender:get_player_name()) then return end
+		end
+
 		if fields.sort or (data.autosort and fields.quit and meta:get_int("autosort") == 1) then
 			sort_inventory(meta:get_inventory())
 		end