Fix a mem leak in CMS

The function CMS_RecipientInfo_set0_pkey() is a "set0" and therefore
memory management passes to OpenSSL. If the same function is called again
then we should ensure that any previous value that was set is freed first
before we set it again.

Fixes #5052

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6142)