[bug-fix] Fix a determinism bug with the DUBT

[bug-fix] Fix a determinism bug with the DUBT

The DUBT can be non-deterministic if an index is equal to
`ZSTD_DUBT_UNSORTED_MARK`. Ensure that never happens by starting the
indices at 2.

This bug was found by the OSS-Fuzz determinism fuzzer. With this change
the fuzzer test passes. And I've confirmed that this is the root cause,
not just hiding the problem.

Aside: This took me a long time to figure out, because I thought I had
tried this first thing. But, apparantly I messed it up, because when I
was going through it again with @felixhandte, I was pointing out that it
wasn't the case, but it turns out it was.

Credit to: OSS-Fuzz