jxl-oxide: `FrameBuffer::new` creates out-of-bounds slices on overflow

Summary

jxl-oxide exposes a public safe API that can construct an undersized FrameBuffer due to unchecked usize multiplication, which immediately trigger panic while initializing the buffer in normal decoding path.

Additionally, calling the safe grouped buffer accessors afterward can create invalid oversized slices from a much smaller allocation, causing undefined behavior; however normal decoding path never reaches UB, because these methods are never used within jxl-oxide.

Impact

On 32-bit platforms this can cause panic by accessing out-of-range indices, making it a DoS vulnerability.