ssl_common.h: fix 'not all control paths return a value' msvc warning

ssl_common.h: fix 'not all control paths return a value' msvc warning

Commit cc5a71637 ("Replace key_scan array of static pointers with inline
function") has introduced get_key_scan() routine with switch statement
which calls ASSERT() in the default case.

Even though "this should never happen", this code triggers Level 1 warning
"not all control paths return a value" in msvc.

Fix by adding "return NULL; /* NOTREACHED */" after ASSERT.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20201217224834.160-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21373.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>