If a stream has no start time, but the first packet has a 'pts', use that

If a stream has no start time, but the first packet has a 'pts', use that
pts to set the start_time.
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 10285 to svn://svn.ffmpeg.org/ffmpeg/trunk