Gamma sanity checks

(1) Make load_gammas() return false on invalid gammas.
(2) Assert that gamma always exists after loading.
(3) Avoid dividing by zero.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2043803002

Review-Url: https://codereview.chromium.org/2043803002
2 files changed