Fix GrAAConvexTessellator trying to fan a 0 point ring
Fix GrAAConvexTessellator trying to fan a 0 point ring

This prevents accessing off an empty array.

Bug: chromium:830038
Change-Id: I5ad019d56659c667564efc498dcef7fb112da4af
Reviewed-on: https://skia-review.googlesource.com/123800
Commit-Queue: Adrienne Walker <c606cbc33c713bf955e452a851d4ad00178ea9fa@chromium.org>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Auto-Submit: Adrienne Walker <c606cbc33c713bf955e452a851d4ad00178ea9fa@chromium.org>
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

aosp-mirror/platform_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,rubenvb/skia,HalCanary/skia-hc,google/skia,google/skia,rubenvb/skia,rubenvb/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,google/skia,google/skia,google/skia,HalCanary/skia-hc