hardening: add insurance to exit on a failed ASSERT()

hardening: add insurance to exit on a failed ASSERT()

The code behind our ASSERT() macro is pretty complex.  Although it seems
to be correct, make it trivially clear we will never return from a failed
assert by adding an _exit(1) call.  As was suggested by Sebastian Krahmer
of the SuSE security team.

To make sure they that tools like clang static analyzer and coverity
understand that assert_failed() will not return, add an
__attribute__((__noreturn__)) annotation.

v2: use __attribute__ instead of inline to convince static analysers.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1445414886-11052-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10349
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit e8a9e3203bf00605dae000d31095076ae038491c)