MdeModulePkg/DxeCore: Avoid accessing non-owned memory

MdeModulePkg/DxeCore: Avoid accessing non-owned memory

The patch fixes two kinds of bugs in DxeCore that accesses memory
which might be freed or owned by other modules.
The two bugs don't cause functionality issue.

1. CoreValidateHandle() checks whether the handle is valid by
   validating its signature. The proper way is to check whether
   the handle is in the handle database.
2. CoreDisconnectControllersUsingProtocolInterface() and
   CoreOpenProtocol() de-reference Link pointer which is
   already freed. The proper way is to not de-reference the pointer.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>