This was caught by ASAN.
Our algorithm header has many one-liners that compute the "next power of
two length / 2" but they all have the caveat that if the input is
already POT, then the "/ 2" part does not occur.
Usually we deal with this by testing the difference against zero.
However in `partition_point` we were skipping the test, thus causing a
potential out-of-bounds access.
I fixed `partition_point` and added a few more tests for non-POT cases.
