Don't free the screen resources reply until we absolutely have to.

On my system, premature free'ing resulted in an error at the xcb_randr_get_output_info_reply call, so randr didn't work at all.

I can provide a bit more info about this, if necessary - the fix does seem very strange, but I assure it has fixed things for me. It was very difficult to debug, and I still don't really understand how it happened - I imagine the config_timestamp and rres_reply structs are sharing some internal info, or something like that.