avformat/matroskaenc: Don't use NULL for %s format string

The argument pertaining to a printf %s conversion specifier must not
be NULL, even if the precision (i.e. the number of characters to write)
is zero. If it is NULL, it is undefined behaviour.
Signed-off-by: NAndreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroskaenc.c

@@ -2118,17 +2118,19 @@ static int mkv_write_vtt_blocks(AVFormatContext *s, AVIOContext *pb, const AVPac
     mkv_track *track = &mkv->tracks[pkt->stream_index];
     ebml_master blockgroup;
     int id_size, settings_size, size;
-    uint8_t *id, *settings;
+    const char *id, *settings;
     int64_t ts = track->write_dts ? pkt->dts : pkt->pts;
     const int flags = 0;
 
     id_size = 0;
     id = av_packet_get_side_data(pkt, AV_PKT_DATA_WEBVTT_IDENTIFIER,
                                  &id_size);
+    id = id ? id : "";
 
     settings_size = 0;
     settings = av_packet_get_side_data(pkt, AV_PKT_DATA_WEBVTT_SETTINGS,
                                        &settings_size);
+    settings = settings ? settings : "";
 
     size = id_size + 1 + settings_size + 1 + pkt->size;