fixed avi RIFF header size

Originally committed as revision 56 to svn://svn.ffmpeg.org/ffmpeg/trunk

libav/avienc.c

@@ -340,7 +340,7 @@ static int avi_write_trailer(AVFormatContext *s)
         /* update file size */
         file_size = url_ftell(pb);
         url_fseek(pb, 4, SEEK_SET);
-        put_le32(pb, file_size);
+        put_le32(pb, file_size - 8);
         url_fseek(pb, file_size, SEEK_SET);
     }
     put_flush_packet(pb);