reformat_slice: Handle an empty slice caused by stop.

Just as a stop value of 0 for a slice with a positive step size will
result in an empty slice, a stop value of -1 or positive equivalent
(given the length is known) will result in an empty slice given a
negative step size. This change handles that case and converts the slice
to a trivial empty slice to make it easier to recognize.

kenjutsu/kenjutsu.py

@@ -120,6 +120,9 @@ def reformat_slice(a_slice, a_length=None):
     elif (step > 0) and (stop == 0):
         start = stop = 0
         step = 1
+    elif (step < 0) and (stop == -1):
+        start = stop = 0
+        step = 1
     elif stop_i and (start >= 0) and (stop >= 0):
         if (step > 0) and (start > stop):
             start = stop = 0