cfg: do not crash when arg is not available
cfg: do not crash when arg is not available

Fixes #852.

keplerproject/luarocks,luarocks/luarocks,tarantool/luarocks,keplerproject/luarocks,tarantool/luarocks,tarantool/luarocks,luarocks/luarocks,keplerproject/luarocks,keplerproject/luarocks,luarocks/luarocks