Set AVSubtitle format to 0. Neither dvdsubdec nor xsubdec intializes format

Set AVSubtitle format to 0. Neither dvdsubdec nor xsubdec intializes format
to 0, but ffplay checks it before displaying subtitles (ffplay.c:1437).
patch by Björn Axelsson, gecko acc.umu se

Originally committed as revision 17170 to svn://svn.ffmpeg.org/ffmpeg/trunk