[issue2266] add null checking to mmst

[qrtt1]

qrtt1 <chingyichan...@gmail.com> added the comment:

Thanks for feedbacks.

I re-check it and find the reason why avio.c call read() after close(). 
because the mmst.c must set an error before jumping to fail.

I create a new patch(mmst_2.patch) without tab char. Please review again.

________________________________________________
FFmpeg issue tracker <iss...@roundup.ffmpeg.org>
<https://roundup.ffmpeg.org/issue2266>
________________________________________________
Index: libavformat/mmst.c
===================================================================
--- libavformat/mmst.c	(revision 25348)
+++ libavformat/mmst.c	(working copy)
@@ -547,8 +547,10 @@
     err = mms_safe_send_recv(mmst, NULL, SC_PKT_ASF_HEADER);
     if (err)
         goto fail;
-    if((mmst->incoming_flags != 0X08) && (mmst->incoming_flags != 0X0C))
+    if((mmst->incoming_flags != 0X08) && (mmst->incoming_flags != 0X0C)) {
+        err = AVERROR_NOFMT;
         goto fail;
+    }
     err = ff_mms_asf_header_parser(mms);
     if (err) {
         dprintf(NULL, "asf header parsed failed!\n");