downloader: prepare_next_transfer: improve error handling for HTTPHEADER
downloader: prepare_next_transfer: improve error handling for HTTPHEADER

1. Print our out of memory message (then abort, as before).

2. To be consistent with the other calls, assert that curl_easy_setopt()
suceeds.  (It doesn't copy the list of headers; it's only supposed to fail
if the protocol is not HTTP).

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