Gcc 9 -O3 fix

Gcc 9 needs some convincing that sopreprbuf really is going to fill
in iov in the call from soreadbuf, even though the failure case
shouldn't happen.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190415121740.9881-1-dgilbert@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>