Backport of the pieces of trunk rev 46589 relevant to

Backport of the pieces of trunk rev 46589 relevant to
fixing an unlikely crash bug in dict resizing, SF
bug 1456209.

The rest of rev 46589 changes whether Python suppresses
exceptions during some dict-related comparisons.  While I
think that's a good idea, it does change visible behavior at
times, and there was already some complaining about that on
the trunk.  Not a good idea for backporting.  The part of
46589 checked in here can at worst stop segfaults, and I doubt
anyone will gripe about that ;-)