Make use of safe file writing in auth handler (fixes #6576)

builtin/game/auth.lua

@@ -42,16 +42,15 @@ local function save_auth_file()
		assert(type(stuff.privileges) == "table")
		assert(stuff.last_login == nil or type(stuff.last_login) == "number")
	end
	local file, errmsg = io.open(core.auth_file_path, 'w+b')
	if not file then
		error(core.auth_file_path.." could not be opened for writing: "..errmsg)
	end
	local content = ""
	for name, stuff in pairs(core.auth_table) do
		local priv_string = core.privs_to_string(stuff.privileges)
		local parts = {name, stuff.password, priv_string, stuff.last_login or ""}
		file:write(table.concat(parts, ":").."\n")
		content = content .. table.concat(parts, ":") .. "\n"
	end
	if not core.safe_file_write(core.auth_file_path, content) then
		error(core.auth_file_path.." could not be written to")
	end
	io.close(file)
end

read_auth_file()