NetworkPkg/HttpDxe: sanity-check the TlsCaCertificate variable before use
In TlsConfigCertificate(), make sure that the set of EFI_SIGNATURE_LIST
objects that the platform stored to "TlsCaCertificate" is well-formed.
In addition, because HttpInstance->TlsConfiguration->SetData() expects
X509 certificates only, ensure that the EFI_SIGNATURE_LIST objects only
report X509 certificates, as described under EFI_CERT_X509_GUID in the
UEFI-2.7 spec.