[opt] Fix too short of match getting generated

[opt] Fix too short of match getting generated

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