Fix downward slices for positive base arrays

[BCSharp]

Fixes issue reported by @slozier in #1828 (review):

Unrelated to this PR, but noticed a slight asymmetry in FixSlice where it does ostart < lb vs ostop < 0. Don't think it hurts anything but seemed odd.

The problem was happening in some cases for arrays with base >= 2 and a negative slice step.

>>> from System import *
>>> arr = Array.CreateInstance(Int32, (3,), (2,))       
>>> arr[-1:-5:-1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: Index was outside the bounds of the array.