Fix two problems (no idea how this ever worked):

Fix two problems (no idea how this ever worked):
- the return value of url_open_dyn_*buf() is 0 on success, so using
   if (!(res = url_open_dyn_*buf())) return res; is not going to work
- url_open_dyn_packet_buf actually writes the max_packet_size before
   each piece of data. Feeding this to the ASF demuxer will never work.
   Therefore, use url_open_dyn_buf() instead.

Originally committed as revision 21853 to svn://svn.ffmpeg.org/ffmpeg/trunk