change convex scan converter to be defensive

ran convex_path_* nanobench, with no appreciable perf loss
(for the cases tested)

Bug: 899689
Bug: skia:8606
Change-Id: Ida253c5057f38b90cda86a1717a8bb7c4a6155dc
Reviewed-on: https://skia-review.googlesource.com/c/175832
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>