fix a minor inefficiency in compress_superblock

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.