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)

Change-Id: I44063c9cb2440a5126105139219aac616d948a01
3 files changed