Update code to ensure the pointer ‘CurrentName’ in function ‘PerformSingleMappingDisplay’ isn’t null before being processed.

Signed-off-by: Shumin Qiu <shumin.qiu@intel.com>
Reviewed-by: Jaben Carsey <Jaben.carsey@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/ShellPkg@14990 6f19259b-4bc3-4df7-8a09-765794883524