[FFmpeg-cvslog] avformat/matroskaenc: Don't use NULL for %s format string

[Andreas Rheinhardt]

ffmpeg | branch: release/3.3 | Andreas Rheinhardt <andreas.rheinhardt at gmail.com> | Sun Jun 14 03:35:41 2020 +0200| [7c50575dcf4ac5c4c030569fe396afb4c1aba587] | committer: Andreas Rheinhardt

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: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
(cherry picked from commit 6de6ce7bc80e874099895b6c73977bc2efb06a4d)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=7c50575dcf4ac5c4c030569fe396afb4c1aba587
---

 libavformat/matroskaenc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
index 3af82a81ba..6689d138af 100644
--- a/libavformat/matroskaenc.c
+++ b/libavformat/matroskaenc.c
@@ -2168,17 +2168,19 @@ static int mkv_write_vtt_blocks(AVFormatContext *s, AVIOContext *pb, AVPacket *p
     MatroskaMuxContext *mkv = s->priv_data;
     ebml_master blockgroup;
     int id_size, settings_size, size;
-    uint8_t *id, *settings;
+    const char *id, *settings;
     int64_t ts = mkv->tracks[pkt->stream_index].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;