end of the UINT_MAX items we can support.

Found by adding a lot of asserts and brute force testing, both
of which I have left in.

Fixes       #967

May also be relevant to #827
(cherry picked from commit 726d93ff5a815774d7a3b4a23dcba2efb3c08ca7
and a6ddafc0e87bcec71f016b8cae77b78641e617d7)