The optimal parser with LDM enabled using minMatch > 3 could generate a match
length of 3 when minMatch >= 4. This is not allowed.
1. Fix the bug
2. Add validation logic to `ZSTD_buildSeqStore()` in debug mode for all block
compressors that checks we never generate too short a match. This way we don't
rely on the `generate_sequences` fuzzer to find this issue.
Credit to OSS-Fuzz
1548bfc349