avformat/matroskadec: Check the return code from strftime()

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

libavformat/matroskadec.c

@@ -1509,7 +1509,7 @@ static void matroska_metadata_creation_time(AVDictionary **metadata, int64_t dat
     time_t creation_time = date_utc / 1000000000 + 978307200;
     struct tm *ptm = gmtime(&creation_time);
     if (!ptm) return;
-    strftime(buffer, sizeof(buffer), "%Y-%m-%d %H:%M:%S", ptm);
+    if (strftime(buffer, sizeof(buffer), "%Y-%m-%d %H:%M:%S", ptm))
         av_dict_set(metadata, "creation_time", buffer, 0);
 }