Make sure that if a stream has a duration but the container doesn't

then the container duration is set from the stream duration.

patch by Neil Brown: [ neilb suse de]
original thread: [FFmpeg-devel] [patch 3/3] Make timing calculations
less dependant on start_time being defined.
date: 08/16/2007 08:27 AM

Originally committed as revision 10408 to svn://svn.ffmpeg.org/ffmpeg/trunk