fix a minor inefficiency in compress_superblock

and in `decodecorpus`:
the specific case `nbSeq=127` can be represented using the 1-byte format.
Note that both the 1-byte and the 2-bytes formats are valid to represent this case,
so there was no "error", produced data remains valid,
it's just that the 1-byte format is more efficient.

fix #3667

Credit to @ip7z for finding this issue.

tests/decodecorpus.c

@@ -825,7 +825,7 @@ static size_t writeSequences(U32* seed, frame_t* frame, seqStore_t* seqStorePtr,
 
     /* Sequences Header */
     if ((oend-op) < 3 /*max nbSeq Size*/ + 1 /*seqHead */) return ERROR(dstSize_tooSmall);
-    if (nbSeq < 0x7F) *op++ = (BYTE)nbSeq;
+    if (nbSeq < 128) *op++ = (BYTE)nbSeq;
     else if (nbSeq < LONGNBSEQ) op[0] = (BYTE)((nbSeq>>8) + 0x80), op[1] = (BYTE)nbSeq, op+=2;
     else op[0]=0xFF, MEM_writeLE16(op+1, (U16)(nbSeq - LONGNBSEQ)), op+=3;