MdeModulePkg: Fix assert in iSCSI.

The bug is caused by using already freed memory.
If there is already an attempt and execute
'reconnect -r' command, all the ConfigFormEntry structure
will be freed in IScsiDriverBindingStop, 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: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>

MdeModulePkg/Universal/Network/IScsiDxe/IScsiConfig.c

@@ -1076,6 +1076,7 @@ IScsiConfigUpdateForm (
     mNumberOfIScsiDevices--;
     RemoveEntryList (&ConfigFormEntry->Link);
     FreePool (ConfigFormEntry);
+    mCallbackInfo->Current = NULL;
   }
   //
   // Allocate space for creation of Buffer