Fix undefined behavior in normalize_t_s

BUG=chromium:743617

Change-Id: I00ad3103cdd5b7d2eac3b6827a3c2932009042a9
Reviewed-on: https://skia-review.googlesource.com/65860
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
1 file changed