Add more check for the code to make it more safely.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>


git-svn-id: https://svn.code.sf.net/p/edk2/code/branches/UDK2014.SP1@16130 6f19259b-4bc3-4df7-8a09-765794883524