Add a missing check on s->s3->tmp.pkey

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7405)

(cherry picked from commit 61bef9bde09dc6099a7c59baa79898e3b003fec3)