Postpone allocation of STACK internal storage ... until a first push(),

insert() or an explicit call to OPENSSL_sk_reserve

Factorise STACK item deletion code

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4379)