UefiCpuPkg/PiSmmCpuDxeSmm: fix error handling

ASSERT() is not proper handling of allocation failures, it gets compiled
out on RELEASE builds.  Print a message and enter dead loop instead.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>