avoid seeking to the end, as it confuses some crappy code

Originally committed as revision 2996 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avio.c

@@ -147,7 +147,7 @@ offset_t url_filesize(URLContext *h)
     offset_t pos, size;
     
     pos = url_seek(h, 0, SEEK_CUR);
-    size = url_seek(h, 0, SEEK_END);
+    size = url_seek(h, -1, SEEK_END)+1;
     url_seek(h, pos, SEEK_SET);
     return size;
 }