curl: Ensure all informationals are checked for completion

According to the documentation, the correct way to ensure all
informationals have been returned by curl_multi_info_read is to loop
until it returns NULL.

Signed-off-by: Matthew Booth <mbooth@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
