[skottie] Avoid UB while parsing 1-based indices

Current impl can underflow int due to unchecked arithmetic.

Add a Parse<size_t> specialization and convert call sites which use
inline arithmetic.  Underflowing unsigned types should be well defined
and caught in later tests.

Bug: oss-fuzz:9798
Change-Id: Iaebe8aad4009e2511fe1d8733d336f5f119bb384
Reviewed-on: https://skia-review.googlesource.com/146648
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed