add 0->0 path to approx_powf()

The path involving approx_log2() and approx_pow2() does not produce 0.
And it's probably not a good idea to think about what approx_log2(0) is
anyway.

Change-Id: If5f48298c5bd5565ae808ebdfbd02649f4dd3046
Reviewed-on: https://skia-review.googlesource.com/85840
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed