MdeModulePkg/NonDiscoverable: fix memory override bug

MdeModulePkg/NonDiscoverable: fix memory override bug

For PciIoPciRead interface, memory prior to Buffer would be written
with zeros if Offset was larger than sizeof (Dev->ConfigSpace), which
would cause serious system exception.

So we add a pre-check branch to avoid memory override.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>