[ByGh00st]

Summary

Hardens BmpDecoder against runtime panics by:

1. Rejecting non-8-bit palettized formats when indexed color decoding is requested.
2. Enforcing strictly positive image dimensions (width > 0 and height != 0).

Details

• Indexed Color Panics: When set_indexed_color(true) was enabled on sub-byte palettized formats (1-bit, 2-bit, or 4-bit BMP), row.clone_from_slice(&indices[0..width]) panicked with an out-of-bounds slice access. The decoder now returns ImageError::Unsupported.
• Zero Dimensions Panics: Images specifying height == 0 or width <= 0 were previously accepted during header parsing. Downstream row arithmetic ((height - 1).try_into().unwrap()) resulted in a TryFromIntError unwrap panic. Headers with non-positive dimensions are now rejected cleanly during header validation.

Testing

• Added regression test test_indexed_color_sub_byte_rejected
• Added regression test test_bmp_zero_height_rejected
• All 31 BMP codec unit tests pass cleanly.

Backward Compatibility

• Zero breaking changes to public APIs.
• Valid BMP images decode identically.