Prevent UBSAN from complaining too early about invalid enums

This is making reproducing https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=15052
difficult.

Docs-Preview: https://skia.org/?cl=217457
Change-Id: I131e8c1b64f621178696b94b63bd09513ca6c170
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/217457
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
3 files changed