lottie: harden malformed asset handling

- Validate gradient color stop arrays before merging them and ignore an
  incomplete trailing alpha stop to prevent out-of-bounds reads.

- Check text child types before casting and centralize owned child cleanup
  so malformed text layers are discarded without invalid casts or leaks.

issues:
- GHSA-58fm-6wm4-6xp6
- GHSA-wcpm-fj7g-q6fg
- GHSA-pc87-qp47-c96r
- GHSA-88w8-mjr4-5hw5