benchmark

StatisticsMedian: Fix bug

Previously, this could return the wrong result when there
was an even number of elements.

There were two `nth_element` calls.  The second call could
change elements in `[center2, end])`, which was where
`center` pointed.  Therefore, `*center` sometimes had the
wrong value after the second `nth_element` call.

Rewrite to use `max_element` instead of the second call to
`nth_element`.  This avoids modifying the vector.