seek back at the end of file after updating header

Originally committed as revision 7473 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/swf.c

@@ -691,6 +691,7 @@ static int swf_write_trailer(AVFormatContext *s)
         put_le32(pb, file_size);
         url_fseek(pb, swf->duration_pos, SEEK_SET);
         put_le16(pb, video_enc->frame_number);
+        url_fseek(pb, file_size, SEEK_SET);
     }
 
     av_free(swf->audio_fifo);