set avi fsize to INT64_MAX if riff tag end is not set and file size is not available

Originally committed as revision 18052 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avidec.c

@@ -260,7 +260,7 @@ static int avi_read_header(AVFormatContext *s, AVFormatParameters *ap)
 
     avi->fsize = url_fsize(pb);
     if(avi->fsize<=0)
-        avi->fsize= avi->riff_end;
+        avi->fsize= avi->riff_end == 8 ? INT64_MAX : avi->riff_end;
 
     /* first list tag */
     stream_index = -1;