UefiCpuPkg/CpuDxe: Fix assert issue on IA32 platform

UefiCpuPkg/CpuDxe: Fix assert issue on IA32 platform

This patch is to fix an assert issue during booting IA32 platforms
such as OvmfIa32 or Quark. This issue is caused by trying to access
page table on a platform without page table. A check is added to
avoid the assert.

Bug tracker: https://bugzilla.tianocore.org/show_bug.cgi?id=724

Cc: Star Zeng <star.zeng@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>