MdeModulePkg/BMMUiLib: Set Handle to NULL after uninstall protocol

MdeModulePkg/BMMUiLib: Set Handle to NULL after uninstall protocol

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1728

Currently Emulator meets ASSERT when enter setup->Continue->enter setup.
When re-enter setup, the BmmDriverHandle in BMMUiLib Constructor
is not NULL which cause InstallMultipleProtocolInterfaces failure,
then ASSERT. So here set BmmDriverHandle to NULL after uninstalling
protocols on it in Destructor function to avoid this issue.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>