NetworkPkg/IScsiDxe: Set ExitBootServiceEvent to NULL after close it.

NetworkPkg/IScsiDxe: Set ExitBootServiceEvent to NULL after close it.

v2:
* Refine the commit log.

There are two place to close the ISCSI ExitBootServiceEvent:
#1.IScsiOnExitBootService(), which is the callback function of
   ExitBootServiceEvent.
#2.IScsiCleanDriverData(), which will be invoked by ISCSI driver
   binding stop().

So, the ExitBootServiceEvent will be closed and freed when exit boot
server is triggered. But it may be closed and freed again in ISCSI driver
binding stop(), which will result in the issue recorded at
https://bugzilla.tianocore.org/show_bug.cgi?id=742.

This patch is to resolve the issue.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>