Fix SkATan2_255 fuzzer crash

Test for degenerate values after computing the ratio, instead of
attempting to catch all tricky cases upfront.

BUG=skia:6511

Change-Id: I8e3421675994dd68a1eff1af3f1456917dd1f9e1
Reviewed-on: https://skia-review.googlesource.com/13726
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed