Add NULL checks from master

The big "don't check for NULL" cleanup requires backporting some
of the lowest-level functions to actually do nothing if NULL is
given.  This will make it easier to backport fixes to release
branches, where master assumes those lower-level functions are "safe"

This commit addresses those tickets: 3798 3799 3801.

Reviewed-by: Matt Caswell <matt@openssl.org>