Limit range of operations on Indexes in 32-bit mode

[Cyan4973]

This is a mitigation strategy:
in 32-bit mode, when comparing pointers at very large distances, the result may eventually become negative, since ptrdiff_t is a signed 32-bit value. It happens if the distance is larger than 2 GB, which is technically possible, even if rare.

This patch will reduce the range of possible distances in 32-bit mode to always remain < 2 GB.

Also, in ZSTD_window_update(), the operation is modified to use unsigned type rather than signed, as it seems it could impact the outcome of a following comparison test. I'm unable to tell if this can lead to a bug, or just to a suboptimal decision, but it's suspicious enough that we don't want that to happen.

Ultimately, I would also prefer to rewrite ZSTD_window_update() in a way which doesn't rely on pointer comparisons, and would therefore be less likely to land into UB territory (even if it works). But modifying that part has sprawling consequences across the entire compression code base, hence it's a much larger work, which will require more time.