Fix flag teleport not working with negative co-ords

ctf_flag/gui.lua

@@ -56,10 +56,10 @@ end)
 minetest.register_on_player_receive_fields(function(player, formname, fields)
 	-- Todo: fix security issue here
 	local name = player:get_player_name()
-	if formname=="ctf:flags" then
+	if formname == "ctf:flags" then
 		for key, field in pairs(fields) do
-			local x,y,z = string.match(key, "goto_(%d+)_(%d+)_(%d+)")
-			if x and y and x then
+			local x,y,z = string.match(key, "goto_([%d-]+)_([%d-]+)_([%d-]+)")
+			if x and y and z then
 				player:setpos({ x=tonumber(x), y=tonumber(y), z=tonumber(z) })
 				return true
 			end