Allow NULL for some _free routines.

Based on the description in https://github.com/openssl/openssl/pull/5757,
this re-implements the "allow NULL to be passed" behavior of a number of
xxx_free routines.  I also fixed up some egregious formatting errors
that were nearby.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5761)