added error detection for writing to a constant swizzle mask

Bug: oss-fuzz:14089
Change-Id: Ie5c51c2de8b9a53aebfae19373cc5a3d7d8e594b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209325
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
3 files changed