If importing the certificate and private key is done separately,
they don't associate with each other, so with this you can add
the private key to the certificate that it belongs to. If the
private key is set in this way, then getPrivateKey() call doesn't
look into the database but just returns the private key stored
in the member variable.

Additionally use CERT_DecodeCertFromPackage to import the DER
certificate from the imput as CERT_DecodeDERCertificate doesn't
import a complete certificate and doesn't add it into the
certificate database.

Change-Id: I29876030f167cc5fa6b887f9bfeb0b84622c751e
Reviewed-on: https://gerrit.libreoffice.org/62271
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>