Revert "stack/stack.c: omit redundant NULL checks."

This reverts commit 88393244.

Removing these checks changes the behaviour of the API which is not
appropriate for a minor release. This also fixes a failure in the
fuzz tests when building with no-comp.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6895)