MdePkg PeiServicesLib: Refine the code to avoid error report.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14769 6f19259b-4bc3-4df7-8a09-765794883524