x/image/bmp: panic when reading out of bound palette index

[thatnealpatel]

Decoding a paletted BMP file with an out-of-range palette index
would result in a panic when accessing pixels in the invalid image.
Decoding now correctly returns an error in this case.

This is CVE-2026-42500 and Go issue https://go.dev/issue/79576.

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This was a PRIVATE track issue, tracked in http://b/502107374.

[gopherbot]

Change https://go.dev/cl/781500 mentions this issue: bmp: reject input with invalid palette index