Protect quicksort pivot index from overflow.

Protect quicksort pivot index from overflow.

Mentioned on
  http://en.wikipedia.org/wiki/Quicksort

  Note the left + (right-left)/2 expression. (left + right)/2 would
  seem to be adequate, but in the presence of overflow, can give the
  wrong answer; for example, in signed 16-bit arithmetic, 32000 +
  32000 is not 64000 but -1536, and dividing that number by two will
  give you a new pivotIndex of -768 — obviously wrong.