Fix memory overflow & VariableSize check issue for SetVariable append write.

Signed-off-by: Chao Zhang <chao.b.zhang@intel.com>
Reviewed-by  : Fu Siyuan  <siyuan.fu@intel.com>
Reviewed-by  : Dong Guo   <guo.dong@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14323 6f19259b-4bc3-4df7-8a09-765794883524