actually return bytes read

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@8502 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/gifdec.c

@@ -317,7 +317,7 @@ static int gif_decode_frame(AVCodecContext *avctx, void *data, int *data_size, u
 
     *picture = s->picture;
     *data_size = sizeof(AVPicture);
-    return 0;
+    return s->bytestream - buf;
 }
 
 static int gif_decode_close(AVCodecContext *avctx)