UefiCpuPkg/PiSmmCpuDxeSmm: Fix build failure for VS2012 and GCC49.

UefiCpuPkg/PiSmmCpuDxeSmm: Fix build failure for VS2012 and GCC49.

Code initialized in function can't be correctly detected by build tool.
Add code to clearly initialize the local variable before use it.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Dandan Bi <dandan.bi@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>