UefiCpuPkg/CpuMpPei: fix vs2012 build error

UefiCpuPkg/CpuMpPei: fix vs2012 build error

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1166

Visual Studio 2012 will complain uninitialized variable, StackBase,
in the CpuPaging.c. This patch adds code to init it to zero and
ASSERT check against 0. This is enough since uninit case will only
happen during retrieving stack memory via gEfiHobMemoryAllocStackGuid.
But this HOB will always be created in advance.

Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>