G1: add precondition to set_remainder_to_point_to_start

`set_remainder_to_point_to_start(start, end)` is called in a single place, inside a `if`, which ensures `start < end`. Therefore, the early-return `if (start >= end)` inside the callee is always false.

Similarly, the early-return inside `set_remainder_to_point_to_start_incl` is unnecessary.