avformat/av1dec: fix return value on some code paths

avformat/av1dec: fix return value on some code paths

If avio_read() returns a value of bytes read that's lower than the
expected, return an error instead. And when there are zero bytes in
the prefetch buffer, return 0 in order for the frame merge bsf to
drain all potentially buffered packets.

Missed by mistake when amending and committing 9a7bdb6d71.

Signed-off-by: James Almer <jamrial@gmail.com>