sleuthkit: build fuzzer for the XPRESS (MS-XCA) decompressors

[MP-GOWTHAM]

Adds a libFuzzer target for the XPRESS (MS-XCA) decompressors that were added to Sleuth Kit in sleuthkit/sleuthkit#3531:

• ossfuzz/tsk_xpress_fuzzer.cc (in the sleuthkit repo) drives both variants: byte 0 selects plain LZ77 or LZ77+Huffman, bytes 1-4 carry the uncompressed size for the Huffman variant, the rest is the compressed stream.
• The build is conditional: the fuzzer only builds once tsk/fs/xpress.c exists in the upstream clone, so this PR does not break the project if the codec PR has not landed yet.
• A seed corpus (16 inputs) derived from the MS-XCA test corpus in unit_tests/base/xpress_test_data.h is zipped into the output.

Verified locally: the fuzzer input layout decodes all 16 corpus vectors to their exact sizes (gcc, -Wall -Wextra clean).

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[github-actions]

MP-GOWTHAM is a new contributor to projects/sleuthkit. The PR must be approved by known contributors before it can be merged. The past contributors are: arthurscchan, simsong, joachimmetz, DonggeLiu, inferno-chromium, alsophian

[DavidKorczynski]

can we remove the seed files from this repository? Store them upstream or something similar. Also, let us know when upstream lands the new harness and it would be great if you can get maintainer approval as well.

[MP-GOWTHAM]

All review points addressed in the updated commit (6c4eb79):

1. Seed files removed from this repository - the 16 corpus vectors (and the zip step that packaged them) are gone from the PR. They live upstream in the sleuthkit test suite (unit_tests/base/xpress_test_data.h) and can be reused from there for the seed corpus once the upstream harness lands.
2. Upstream harness status: Add XPRESS (MS-XCA) plain LZ77 and LZ77+Huffman decoders sleuthkit/sleuthkit#3531 (codec) and [poppler] Compile and use openjpeg2 #3532 (ossfuzz/tsk_xpress_fuzzer.cc harness) are both still open. The build here stays conditional on tsk/fs/xpress.c being present, so nothing regresses until they land; I will ping here the moment they do.
3. Maintainer approval: I have pinged the sleuthkit maintainers for an approval of the upstream PRs (Propose ClickHouse to oss-fuzz #3531/[poppler] Compile and use openjpeg2 #3532) which serve as the base for this one, and will relay their approval here.