MdeModulePkg/PiSmmCore: Cache CommunicationBuffer info before using it

MdeModulePkg/PiSmmCore: Cache CommunicationBuffer info before using it

gSmmCorePrivate->CommunicationBuffer and gSmmCorePrivate->BufferSize locate at
runtime memory region. That means they could be modified by non-SMM code during
runtime.

We should cache them into SMM local variables before we verify them. After
verification, we should use the cached ones directly instead of the ones in
gSmmCorePrivate.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>