curl: Fix hang reading from slow connections

When receiving a new aio read request, we first look for an existing
transaction whose range will cover the read request by the time it
completes. However, we weren't checking that the existing transaction
was still active. If it had timed out, we were adding the request to a
transaction which would never complete and had already been cancelled,
resulting in a hang.

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>
