Add NULL checks where ContentInfo data can be NULL

Add NULL checks where ContentInfo data can be NULL

PKCS12 structures contain PKCS7 ContentInfo fields. These fields are
optional and can be NULL even if the "type" is a valid value. OpenSSL
was not properly accounting for this and a NULL dereference can occur
causing a crash.

CVE-2024-0727

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23361)