In CircuitPython (only), a the slice assignment to [-1👎-3] of an
ndarray of length 1 caused a crash. This appears to be because in
CircuitPython, the internal pointer of an empty array was NULL rather than
pointing at some memory which happened to be valid and assignable.
This appears to be a corner case of how integer promotion rules work in C.
The expression `slice.stop - slice.start + (slice.step + correction)`
is type `unsigned long` and on a LP64 platform its value is
18446744073709551614. This led to the function returning that this slice
had length 1 instead, during the automated tests.
By casting to signed types capable of holding indices and sizes, the
problem is corrected on both platforms.
Jeff Epler
3dc52575f0