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

libavformat/rtp_asf.c

@@ -196,7 +196,7 @@ static int asfrtp_parse_packet(AVFormatContext *s, PayloadContext *asf,
                 av_free(p);
             }
             if (!len_off && !asf->pktbuf &&
-                !(res = url_open_dyn_packet_buf(&asf->pktbuf, rt->asf_ctx->packet_size)))
+                (res = url_open_dyn_buf(&asf->pktbuf)) < 0)
                 return res;
             if (!asf->pktbuf)
                 return AVERROR(EIO);