This works around the problem noted by Yamamoto Takashi in bug #5906,
that there were code paths whereby we could reach AtCleanup_Portals
with a portal's cleanup hook still unexecuted.  The changes I made
a few days ago were intended to prevent that from happening, and
I think that on balance it's still a good thing to avoid, so I don't
want to remove the Assert in AtCleanup_Portals.  Hence do this instead.