curl: Fix return from curl_read_cb with invalid state

A curl write callback is supposed to return the number of bytes it
handled.  curl_read_cb would have erroneously reported it had handled
all bytes in the event that the internal curl state was invalid.

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>