avformat/oggparseopus: fix segmented timestamps

Fix timestamp calculation for code 3 Ogg Opus packets with less than
2 bytes in the last segment (e.g. packet length 255 or 256).
A sample that would seek incorrectly in ffplay can be created with:
  ffmpeg -i in.wav -b:a 34k -vbr off -frame_duration 60 out.opus
and libopus 1.1

Also do not read past the end of the buffer when a packet has length 0.

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