Determinism: Avoid Mapping Window into Reserved Indices during Reduction

Determinism: Avoid Mapping Window into Reserved Indices during Reduction

PR #2850 attempted to fix a determinism bug that was uncovered by OSS-Fuzz. It
succeeded in addressing that source of non-determinism, but introduced a new
one: it was possible, when index reduction occurred, to map indices in the
window to the reserved value, which would cause them to be zeroed, potentially
altering parsing of the input.

This PR addresses this issue. It makes sure that the bottom of the window is
always `>= ZSTD_WINDOW_START_INDEX`.

I'm not sure if this makes #2850 redundant. I think it's probably still
valuable to have that protection as well.

Credit to OSS-Fuzz for discovering this issue.