Correct opts calulation in ffmpeg.c.
This correct the stop point for demuxing with -vcodec copy and -t as well as
packet interleaving. (we already diddrop packets but kept demuxing them
for too long due to opts being wrong)
the change to ffm is due to 2 packets with timestamp 0 being stored
in different order.
Originally committed as revision 21626 to svn://svn.ffmpeg.org/ffmpeg/trunk