[skottie] Fix OOB access in Parse<SkPoint>

SkJSON requires valid array indices, so callers must guard against
out-of-bounds conditions explicitly.

Bug: oss-fuzz:8956
Change-Id: I50b96b088e44a4c1a569e6911d4be5d75799b464
Reviewed-on: https://skia-review.googlesource.com/135445
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
4 files changed