interactive.c: add the upper bound for startupdata size

interactive.c: add the upper bound for startupdata size

The size is passed from the limited-privileges process.
This check ensures that the service won't allocate
more than needed.

Reported-by: Joshua Rogers <contact@joshua.hu>
Found-by: ZeroPath (https://zeropath.com/)
Change-Id: I75ebf01641db4dcd07041e3b8b3fa8a632d07595
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1331
Message-Id: <20251030150432.4689-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg34039.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>