err/err.c: add err_clear_last_constant_time.

Expected usage pattern is to unconditionally set error and then
wipe it if there was no actual error.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit f658a3b64d8750642f4975090740865f770c2a1b)