check unpremul scale directly against infinity

There are non-zero values of a that make infinite 1.0f/a.
Let's just check for the real thing we care about, that
scale is finite.

Bug: skia:7123

Change-Id: If97574c9f3f2f0b73c749d0bea9aa19e6114f4d1
Reviewed-on: https://skia-review.googlesource.com/58460
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed