matroskaenc: Don't set language to empty string, use "und"

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

libavformat/matroskaenc.c

@@ -737,7 +737,7 @@ static int mkv_write_track(AVFormatContext *s, MatroskaMuxContext *mkv,
         put_ebml_string(pb, MATROSKA_ID_TRACKNAME, tag->value);
     tag = av_dict_get(st->metadata, "language", NULL, 0);
     if (mkv->mode != MODE_WEBM || codec->codec_id != AV_CODEC_ID_WEBVTT) {
-        put_ebml_string(pb, MATROSKA_ID_TRACKLANGUAGE, tag ? tag->value:"und");
+        put_ebml_string(pb, MATROSKA_ID_TRACKLANGUAGE, tag && tag->value ? tag->value:"und");
     } else if (tag && tag->value) {
         put_ebml_string(pb, MATROSKA_ID_TRACKLANGUAGE, tag->value);
     }