Fix panic in slice filter with negative length

[ChrisJr404]

Summary

The slice filter panics with runtime error: slice bounds out of range when given a negative length. This is reachable from the main entry points (Engine.ParseAndRender, Template.Render) purely through template source, so a template author can crash the renderer.

Cause

slice computes end := start + length and only clamps end against the upper bound (len). It never clamps against start. A negative length makes end < start, and the subsequent runes[start:end] / slice[start:end] slice expression panics.

Minimal reproductions (both panic before this change):

eng := liquid.NewEngine()
eng.ParseAndRenderString(`{{ "Liquid" | slice: 2, -1 }}`, nil)          // string case
eng.ParseAndRenderString(`{{ "a,b,c" | split: "," | slice: 1, -1 }}`, nil) // array case

Fix

Clamp end up to start in both the string and array branches, so a negative length yields an empty result instead of panicking. This matches Ruby's String#slice(start, length), which returns nil for a negative length. Existing valid slices (positive length, negative start, out-of-range values) are unaffected.

Tests

Added regression cases to filterTests covering negative length for both a string (ASCII and multi-byte runes) and an array. make test passes.

Checklist

• I have read the contribution guidelines.
• make test passes.
• make lint passes. (golangci-lint not run locally; go vet ./... is clean and the change is a two-line bounds clamp.)
• New and changed code is covered by tests.
• Performance improvements include benchmarks. (N/A)
• Changes match the documented behavior of Shopify (negative length yields an empty result).