Fixes #8923

Found using the openssl cms -resign option.
This uses an alternate path to do the signing which was not adding the required signed attribute
content type. The content type attribute should always exist since it is required is there are
any signed attributes.
As the signing time attribute is always added in code, the content type attribute is also required.
The CMS_si_check_attributes() method adds validity checks for signed and unsigned attributes
e.g. The message digest attribute is a signed attribute that must exist if any signed attributes
exist, it cannot be an unsigned attribute and there must only be one instance containing a single
value.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8944)

(cherry picked from commit 19e512a8244a6f527d0194339a8f9fc45468537a)