It looks like it's possible to divide by zero safely in the one place
it's used, so I've replaced it with sk_ieee_float_divide().
Change-Id: I3ebfa697869bc7d4da61b6fc5711421a833c9081
Reviewed-on: https://skia-review.googlesource.com/c/169080
Reviewed-by: Florin Malita <fmalita@chromium.org>