NetworkPkg/HttpDxe: sanity-check the TlsCaCertificate variable before use

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.

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=909
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>