MdeModulePkg/PiSmmCore: SmmEntryPoint underflow (CVE-2021-38578)

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=3387



Added use of SafeIntLib to validate values are not causing overflows or
underflows in user controlled values when calculating buffer sizes.

Signed-off-by: Miki Demeter <miki.demeter@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>