Make parse_date return INT64_MIN in case of unparsable input.

Patch by Stefano Sabatini: [stefano sabatini-lala poste it]
Original thread:
[FFmpeg-devel] [PATCH] Enhace documentation forlibavformat/utils.c:parse_date
Date: 08/17/2007 09:40 PM

Originally committed as revision 10533 to svn://svn.ffmpeg.org/ffmpeg/trunk