Merge commit '3e1c9da38b849ce2982b516004370081fdd89ed0' into release/2.4

* commit '3e1c9da38b849ce2982b516004370081fdd89ed0':
  matroskadec: fix crash when parsing invalid mkv

See: d9fe6b926c
Merged-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -2013,12 +2013,15 @@ static int matroska_read_header(AVFormatContext *s)
     matroska->ctx = s;
 
     /* First read the EBML header. */
-    if (ebml_parse(matroska, ebml_syntax, &ebml) ||
-        ebml.version         > EBML_VERSION      ||
+    if (ebml_parse(matroska, ebml_syntax, &ebml) || !ebml.doctype) {
+        av_log(matroska->ctx, AV_LOG_ERROR, "EBML header parsing failed\n");
+        ebml_free(ebml_syntax, &ebml);
+        return AVERROR_INVALIDDATA;
+    }
+    if (ebml.version         > EBML_VERSION      ||
         ebml.max_size        > sizeof(uint64_t)  ||
         ebml.id_length       > sizeof(uint32_t)  ||
-        ebml.doctype_version > 3                 ||
-        !ebml.doctype) {
+        ebml.doctype_version > 3) {
         av_log(matroska->ctx, AV_LOG_ERROR,
                "EBML header using unsupported features\n"
                "(EBML version %"PRIu64", doctype %s, doc version %"PRIu64")\n",