BUG: fix uninitialized use of size 1 reductions

Size 1 reductions do not intiialize the iterator fully as normal
reductions which triggers uninitialized use in NpyIter_IsFirstVisit.
To fix this check the size of the iterator and return true if it only
has size 1.
Merge pull request #4535 from juliantaylor/itersize-1

BUG: fix uninitialized use of size 1 reductions