zeroize struct image in packet_id_persist_save() before writing to disk

while this really is only a debug function, ensuring that no uninitialized
heap content ends up in padding in the structure and thus to disk is good
practice.

Reported-by: Joshua Rogers <[email protected]>
Found-by: ZeroPath (https://zeropath.com/)

Change-Id: I7f4c7b0ca748975defca1e5104e7077a761cd49c
Signed-off-by: Gert Doering <[email protected]>
Acked-by: Frank Lichtenheld <[email protected]>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1323
Message-Id: <[email protected]>
URL: https://www.mail-archive.com/[email protected]/msg33983.html
Signed-off-by: Gert Doering <[email protected]>

Author: cron2

src/openvpn/packet_id.c

@@ -511,6 +511,7 @@ packet_id_persist_save(struct packet_id_persist *p)
         && (p->time != p->time_last_written || p->id != p->id_last_written))
     {
         struct packet_id_persist_file_image image;
+        CLEAR(image);
         ssize_t n;
         off_t seek_ret;
         struct gc_arena gc = gc_new();