When looking for the last packet in each

When looking for the last packet in each
stream, so as to calculate the duration, don't stop
as soon as all streams have seen at least one packet.
Otherwise the duration will be shorter than it
should be.  We must keep reading to the end-of-file.
patch by neilb suse de

Originally committed as revision 10139 to svn://svn.ffmpeg.org/ffmpeg/trunk