matroskadec: Fix null pointer exception on corrupted input.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -1377,7 +1377,7 @@ static int matroska_read_header(AVFormatContext *s)
     /* First read the EBML header. */
     if (ebml_parse(matroska, ebml_syntax, &ebml)
         || ebml.version > EBML_VERSION       || ebml.max_size > sizeof(uint64_t)
-        || ebml.id_length > sizeof(uint32_t) || ebml.doctype_version > 3) {
+        || ebml.id_length > sizeof(uint32_t) || ebml.doctype_version > 3 || !ebml.doctype) {
         av_log(matroska->ctx, AV_LOG_ERROR,
                "EBML header using unsupported features\n"
                "(EBML version %"PRIu64", doctype %s, doc version %"PRIu64")\n",