Clean the flag before use it to avoid use old error value.

Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14510 6f19259b-4bc3-4df7-8a09-765794883524