[skcms] Harden CLUT indexing, enforce limits, and add test case

- Add validation for grid point counts to enforce SKCMS_MAX_GRID_POINTS during parsing, protecting index calculations from integer overflow.
- Secure clut() by clamping inputs to the [0, 1] range to prevent negative coordinate indexing and out-of-bounds (OOB) memory access.
- Initialize index and weight arrays in clut() to avoid uninitialized memory use.
- Add test case 'test_CLUT_OutOfBoundsInput' in tests.c to verify safe handling of extreme out-of-bounds input values on both B2A and A2B paths.

Fixed: b/513702971
Change-Id: I3c07496be905ff553e9ee56b10eb586e5e839879
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/1241440
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>