Fix FreeBSD builds with WARNS=6 (#1869)

Fix FreeBSD builds with WARNS=6 (#1869)

WARNS=6 on FreeBSD passes several CFLAGS that causes the previous code
to fail with `-Wincompatible-pointer-types-discards-qualifiers` when
compiled with clang.

This particular change adjusts the code to be
`-Wincompatible-pointer-types-discards-qualifiers` clean. This change
changes the calls to use OSSL_PARAM macro abbreviated calls, instead of
calling more verbose (and less documented) callers.

While here, also address a `mac` object leak if `ctx` cannot be
allocated cleanly by always free'ing `mac` after it's been attached to
`ctx`.

Co-authored-by: Pierre Pronchery <pierre@freebsdfoundation.org>
Sponsored by:	The FreeBSD Foundation
Signed-off-by:	Enji Cooper <yaneurabeya@gmail.com>