BUG: add missing gufunc failure return values and missing malloc fail check
BUG: nditer: Initialize buffer reduce pos

The FirstVisit function uses this, but when the reduction is
over a single element it isn't considered a reduction. This is
fine, however the reduce pos must still be initialized to 0 in
that case. Also changes the check order so that it is not
necessary to initialize the outerstrides as well.

See also gh-4134, and gh-4535.
DOC: add size 1 object reduction to release notes
Merge pull request #4540 from juliantaylor/backports

Backports for 1.8