Bugfix candidate.

Bugfix candidate.
Two exceedingly unlikely errors in dictresize():
1. The loop for finding the new size had an off-by-one error at the
   end (could over-index the polys[] vector).
2. The polys[] vector ended with a 0, apparently intended as a sentinel
   value but never used as such; i.e., it was never checked, so 0 could
   have been used *as* a polynomial.
Neither bug could trigger unless a dict grew to 2**30 slots; since that
would consume at least 12GB of memory just to hold the dict pointers,
I'm betting it's not the cause of the bug Fred's tracking down <wink>.