[FFmpeg-cvslog] avformat/aviobuf: Don't use NULL as src for memcpy

[Andreas Rheinhardt]

ffmpeg | branch: master | Andreas Rheinhardt <andreas.rheinhardt at outlook.com> | Wed Sep 28 19:38:01 2022 +0200| [63b31565584e7aaef9c43ca9c0c7a44c6cbc9e97] | committer: Andreas Rheinhardt

avformat/aviobuf: Don't use NULL as src for memcpy

This might happen in avio_write() if size == 0
when the direct codepath is taken. It is undefined behaviour
according to the spec although it happens to work in practice.
Fixes the webm-webvtt-remux FATE-test under UBSan.

Reviewed-by: James Almer <jamrial at gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=63b31565584e7aaef9c43ca9c0c7a44c6cbc9e97
---

 libavformat/aviobuf.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
index b20b1a611a..257535a964 100644
--- a/libavformat/aviobuf.c
+++ b/libavformat/aviobuf.c
@@ -231,12 +231,14 @@ void ffio_fill(AVIOContext *s, int b, int64_t count)
 
 void avio_write(AVIOContext *s, const unsigned char *buf, int size)
 {
+    if (size <= 0)
+        return;
     if (s->direct && !s->update_checksum) {
         avio_flush(s);
         writeout(s, buf, size);
         return;
     }
-    while (size > 0) {
+    do {
         int len = FFMIN(s->buf_end - s->buf_ptr, size);
         memcpy(s->buf_ptr, buf, len);
         s->buf_ptr += len;
@@ -246,7 +248,7 @@ void avio_write(AVIOContext *s, const unsigned char *buf, int size)
 
         buf += len;
         size -= len;
-    }
+    } while (size > 0);
 }
 
 void avio_flush(AVIOContext *s)