UefiCpuPkg/PiSmmCpuDxeSmm: Add check for pointer Pml5Entry

UefiCpuPkg/PiSmmCpuDxeSmm: Add check for pointer Pml5Entry

The pointer Pml5Entry, returned from call to function
AllocatePageTableMemory, may be null.
So add check for it.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>