stack/stack.c: omit redundant NULL checks.

Checks are left in OPENSSL_sk_shift, OPENSSL_sk_pop and OPENSSL_sk_num.
This is because these are used as "opportunistic" readers, pulling
whatever datai, if any, set by somebody else. All calls that add data
don't check for stack being NULL, because caller should have checked
if stack was actually created.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6860)