lavf/prompeg: prompeg_write() must report data all was written

Previously, prompeg_write() would only report to caller that bytes we
written when a FEC packet was actually created.  Not all RTP packets are
expected to generate a FEC packet however, so this behavior was causing
avio to retry writing the RTP packet, eventually forcing the FEC state
machine to send a FEC packet erroneously (and so breaking out of the
retry loop).

This was resulting in incorrect FEC data being generated, and far too
many FEC packets to be sent (~100% FEC overhead).

fix #7863

Signed-off-by: David Holroyd <david.holroyd@m2amedia.tv>
