Prevent overflows when using gamma_alloc_size

BUG=636268
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2230163002

Review-Url: https://codereview.chromium.org/2230163002
diff --git a/AUTHORS b/AUTHORS
index 35cf14c..a5e8965 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -16,6 +16,7 @@
 ARM <*@arm.com>
 Ehsan Akhgari <ehsan.akhgari@gmail.com>
 George Wright <george@mozilla.com>
+GiWan Go <gogil@stealien.com>
 Google Inc. <*@google.com>
 Herb Derby <herbderby@gmail.com>
 Igalia <*@igalia.com>