pow2_rank is undefined for 0.
bucket_of currently works around that by using a bitmask of 0.
To allow asserting that count_{leading,trailing}_zeros are not
called with 0, we want to avoid it at all call sites.
Fixes#4153
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Message-Id: <20190623162137.2401-1-bhalevy@scylladb.com>