change ZSTD_storeSeq() interface to accept matchLength

instead of mlBase.

This removes the need to do `- MINMATCH` at every call site.

The new interface contract is checked with an `assert()`.

tests/decodecorpus.c

@@ -759,7 +759,7 @@ static U32 generateSequences(U32* seed, frame_t* frame, seqStore_t* seqStore,
         }
         /* use libzstd sequence handling */
         ZSTD_storeSeq(seqStore, literalLen, literals, literals + literalLen,
-                      offsetCode, matchLen - MINMATCH);
+                      offsetCode, matchLen);
 
         literalsSize -= literalLen;
         excessMatch -= (matchLen - MIN_SEQ_LEN);