avformat/utils: Add av_assert1 to preclude NULL + len, len != 0

Such a scenario is undefined behaviour and would also indicate a bug
in our code.

Suggested-by: James Almer <jamrial@gmail.com>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit da857099e9)

libavformat/utils.c

@@ -1472,6 +1472,7 @@ static int parse_packet(AVFormatContext *s, AVPacket *pkt,
         pkt->pts = pkt->dts = AV_NOPTS_VALUE;
         pkt->pos = -1;
         /* increment read pointer */
+        av_assert1(data || !len);
         data  = len ? data + len : data;
         size -= len;