hls: skip to next segment if the current is unavailable

hls: skip to next segment if the current is unavailable

Apparently, some live streams can delete segments too early, maybe
because the client is too far behind. In this case, it's better to skip
the segment, instead of returning EOF. (Yes, the HLS demuxer actually
returns AVERROR_EOF if opening the segment returns a 404 HTTP error.)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>