NetworkPkg: Fix assert issue in iSCSI driver

The bug is caused by using already freed memory.
If there is already an attempt and execute
'reconnect -r' command, all the AttemptConfig structure
will be freed, but the mCallbackInfo->Current is not
configured as null and this pointer will be used again in
IScsiFormExtractConfig.

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

NetworkPkg/IScsiDxe/IScsiMisc.c

@@ -900,6 +900,8 @@ EXIT:
 
   gBS->CloseEvent (Private->ExitBootServiceEvent);
 
+  mCallbackInfo->Current = NULL;
+
   FreePool (Private);
   return Status;
 }