Preventing division by 0 in non-separable blend mode shaders.

In the software path, the same issue has been fixed some time ago:
https://codereview.chromium.org/114173002

BUG=skia:

Review URL: https://codereview.chromium.org/666043003
diff --git a/AUTHORS b/AUTHORS
index 51d2bf5..c36ea2c 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -31,3 +31,4 @@
 The Chromium Authors <*@chromium.org>
 Thiago Fransosi Farina <thiago.farina@gmail.com>
 Pavel Krajcevski <pavel@cs.unc.edu>
+Ion Rosca <rosca@adobe.com>