MdeModulePkg: Refine the code in UiApp

Refine the code in UiApp to prevent the potential risk.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>

Reviewed-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18216 6f19259b-4bc3-4df7-8a09-765794883524
2 files changed