rtpdec_asf: Don't free the payload context in the .close function

This was missed in bb4a310b. This fixes a double free on close.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpdec_asf.c

@@ -278,7 +278,6 @@ static void asfrtp_close_context(PayloadContext *asf)
 {
     ffio_free_dyn_buf(&asf->pktbuf);
     av_freep(&asf->buf);
-    av_free(asf);
 }
 
 #define RTP_ASF_HANDLER(n, s, t) \