Fix intset midpoint selection

The classic (min+max)/2 is provably unsafe.  Fixed
as recommended in research:
http://googleresearch.blogspot.com/2006/06/extra-extra-read-all-about-it-nearly.html

Fix inspired by @wjin, but I used a different approach.
(later, I found @kuebler fixed the same issue too).

Fixes #1741, #1602