security/openvpn: assert()->ASSERT() cleanup

This reverts my prior assert -> if...return(false) change
to Ralf Lici's backport, and replaces all assert() by ASSERT().

Gert Doering writes:
| OpenVPN's ASSERT() macro will do a bit more than the standard-libc
| assert() call, namely print out which function and what expression
| failed, before calling _exit(1).  Also, it can not be accidentially
| compiled-away (-DNDEBUG).
|
| Use of ASSERT() generally only advised in cases of "this must not happen,
| but if it does, it's a programming or state corruption error that we
| must know about".  Use of assert() is lacking the extra debug info, and as
| such, not advised at all.

This is a backport of OpenVPN master changes...
Obtained from: 	Gert Doering <gert@greenie.muc.de>
Obtained from:  <https://gerrit.openvpn.net/c/openvpn/+/1171>
PR:             289303
MFH:            2025Q3 (suggestion after 10 days)

(cherry picked from commit d92d41ca)