Guard against integer overflow when classifying transfer functions

Bug: chromium:1016183
Change-Id: I2fa470db3690f79d83fd17b01e421e7457f7c725
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/251246
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed