downloader prepare_next_transfer: fix error message
downloader prepare_next_transfer: fix error message

full_url is not relevant to CURLOPT_ERRORBUFFER.

It's funny I missed this weirdness the first time round, considering the
lifetime of full_url.  But that's no longer significant, because the
full_url variable is marked with a cleanup attribute.

rpm-software-management/librepo,rpm-software-management/librepo,Tojaj/librepo,Tojaj/librepo,Tojaj/librepo,rpm-software-management/librepo