Add libFuzzer target for BrotliDecoderAttachDictionary overflow

[0xazanul]

This PR adds a libFuzzer target for BrotliDecoderAttachDictionary().

The fuzz target exposes an integer overflow in AttachCompoundDictionary()
(c/dec/decode.c:1545), where a size_t value is truncated to int:

addon->total_size += (int)size;

When size > INT_MAX, this results in signed integer overflow and corrupts
addon->total_size.

The corrupted state propagates into
EnsureCompoundDictionaryInitialized(), and under fuzzing leads to
memory corruption and a segmentation fault in
BrotliTransformDictionaryWord().

Reproducibility:

• Triggered using libFuzzer with AddressSanitizer and UBSan enabled
• UBSan reports signed integer overflow
• AddressSanitizer reports a segmentation fault

This fuzz target improves test coverage and enables integration
with OSS-Fuzz to detect this issue and similar regressions.

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[0xazanul]

Is there any update on this one ?

[0xazanul]

Hey just for confirmation the issue i reported is in 1443 not 1438 , can u guys confirm