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

Summary

### 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.