bmp: validate palette indices when decoding paletted images

[mohammadmseet-hue]

Validate that decoded pixel indices are within palette bounds
in decodePaletted. This is the same fix applied to the TIFF
decoder in commit 3bbf4a6 (tiff: Validate palette indices
when parsing palette-color images), but for BMP.

A crafted BMP with a small palette but out-of-range pixel
indices causes a panic when the decoded image is accessed.
For example, a file with colorUsed=2 (2-color palette) but
pixel data containing index 255 causes:

runtime error: index out of range [255] with length 2

The TIFF decoder was patched to reject invalid palette
indices, but the BMP decoder has the identical vulnerable
pattern. The decoder stores the index without validation.
When any consumer calls At(x, y) on the returned
image.Paletted, it panics.

This affects all paletted BMP modes: 1, 2, 4, and 8 bits
per pixel.

The fix validates each pixel index against the palette
length during decoding and returns an error for any
out-of-range index.

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Mohammad Seet:

Patch Set 1:

Done. Updated PR description to remove markdown formatting, wrap lines at ~76 characters, and fix any issues flagged by the bot.

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[gopherbot]

Message from Mohammad Seet:

Patch Set 2:

(1 comment)

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