Fix an integer overflow bug in {size2index,s2u}_compute().

This {bug,regression} was introduced by
155bfa7da18cab0d21d87aa2dce4554166836f5d (Normalize size classes.).

This resolves #241.

Bug: 21633176

(cherry picked from commit dde067264db6b801f7ffae9616a35dba5d2d9ad4)
(cherry picked from commit 8eb485507c98b8efe2611e602269f977f17beaef)

Change-Id: Ib6a6cd096d6e221f68c388ebe5f39571428511cf
3 files changed