Tiny defensive changes:

• avoid exposing invalid heap pointers to FFI if the cache insert fails. The caller would've still received an error and should not expect the pointers to be valid, but it's better to avoid exposing them in this error case.
• init avif_info and avif_image with defaults to match other code and avoid exposing uninitialized structs on error paths
• make sample_info conversion failure explicit rather than silent
• add missing nullptr checks in some FFI functions
• avoid truncating timescale silently
• add an assert to catch read callback misbehaviour