fixed internal error when parsing invalid SkSL code

Bug: skia:
Change-Id: I1c801a26727b72f36d76e1a1c21cd0e571107f8c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228558
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
3 files changed