Properly handle null bytes and invalid characters in control messages

Properly handle null bytes and invalid characters in control messages

This makes OpenVPN more picky in accepting control message in two aspects:
- Characters are checked in the whole buffer and not until the first
  NUL byte
- if the message contains invalid characters, we no longer continue
  evaluating a fixed up version of the message but rather stop
  processing it completely.

Previously it was possible to get invalid characters to end up in log
files or on a terminal.

This also prepares the logic a bit in the direction of having a proper
framing of control messages separated by null bytes instead of relying
on the TLS framing for that. All OpenVPN implementations write the 0
bytes between control commands.

This patch also include several improvement suggestion from Reynir
(thanks!).

CVE: 2024-5594

Reported-By: Reynir Björnsson <reynir@reynir.dk>
Change-Id: I0d926f910637dabc89bf5fa919dc6beef1eb46d9
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20240619103004.56460-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28791.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>