To verify a certificate chain, this function replaces known
certificates with the ones in the system trust store if possible.

However, if it is found, the function checks the validity of the
original certificate rather than the certificate found in the trust
store.  That reveals a problem in a scenario that (1) a certificate is
signed by multiple issuers and (2) one of the issuers' certificate has
expired and included in the input chain.

This patch makes it a little robuster by actually retrieving the
certificate from the trust store and perform check against it.

Signed-off-by: Daiki Ueno <ueno@gnu.org>