[Release-0.6] Fix IntSet overflow with large unsigned ints

[mbauman]

This is already fixed on master through a larger, unrelated change. This is a very minimal patch to correct the bad behavior.

Ref https://discourse.julialang.org/t/intset-crashes-julia-0-6-1/6727/5

[ararslan]

I guess I'll be doing a 0.6.2. 😛

[rfourquet]

@ararslan I can see a couple more bugfixes in master which are not in 0.6.1 (#23557, #23191, #23179), would you like me to backport them before releasing 0.6.2 ?

[ararslan]

@rfourquet My comment was somewhat in jest, though it seems like there are enough things to backport that a 0.6.2 release might make sense. But we'll wait a bit to see whether a 0.6.2 is necessary; depending on the timing of 0.7/1.0, it may not be worth the effort.

[ararslan]

Removing the backport label since this is now in the release-0.6 branch.