UefiCpuPkg/PiSmmCpuDxeSmm: Skip if AllocatedSize is 0

UefiCpuPkg/PiSmmCpuDxeSmm: Skip if AllocatedSize is 0

Needn't to copy register table if AllocatedSize is 0.

v4:
  Fix potential uninitialized variable issue.

v5:
  Set DestinationRegisterTableList[Index].RegisterTableEntry before
  RegisterTableEntry is updated.

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