matroskadec: fix strcmp(NULL)

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -1145,7 +1145,7 @@ static void matroska_convert_tag(AVFormatContext *s, EbmlList *list,
     int i;
 
     for (i=0; i < list->nb_elem; i++) {
-        const char *lang = strcmp(tags[i].lang, "und") ? tags[i].lang : NULL;
+        const char *lang= (tags[i].lang && strcmp(tags[i].lang, "und")) ? tags[i].lang : NULL;
 
         if (!tags[i].name) {
             av_log(s, AV_LOG_WARNING, "Skipping invalid tag with no TagName.\n");