Change another AVERROR(EIO) inot AVERROR_EOF, because it really signals EOF,
not a stream error. This also fixes an infinite loop on EOS during ASF file
playback, and Michael claims he "likes the patch" (ref: [PATCH] RTSP-MS
15/15: move packet_time_start zero value assignment in asf.c" mailinglist
tread).

Originally committed as revision 18552 to svn://svn.ffmpeg.org/ffmpeg/trunk
