This commit adds asserts to ensure that the preconditions on
the points_ list are met when calling the function, namely:
- the list has at least two elements
- the first and last element are at abscissas X_MIN and X_MAX

Moreover, this commit moves the (p != points_.begin()) check
out of the if clause, and into an assert as well.

This ensures that if a bug happens here again, we will catch
it instead of sweeping it under the rug.