The calculation to find the factor between two evaluated points assumed
that the points were not at the same location. This assumption is some-
what reasonable, since we might expect `lower_bound` to skip those
point anyway. However, the report found a case where the first two
evaluated points were coincident, and there is no strong reason not
to make this safe, so add a check for 0 length before the division.
4cbcfd22f5