added error check for invalid 'in' variables

Bug: skia:
Change-Id: I0e767f322ad10a0ca4c6209c516b538ff36c9279
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/233986
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
2 files changed