Fix overflow protection with wlog=31

Fix overflow protection with wlog=31

The overflow protection is broken when the window log is `> (3U << 29)`, so 31.
It doesn't work when `current` isn't around `1U << windowLog` ahead of `lowLimit`,
and the the assertion `current > newCurrent` fails. This happens when the same
context is used many times over, but with a large window log, like in zstdmt.

Fix it by triggering correction based on `nextSrc - base` instead of `lowLimit`.

The added test fails before the patch, and passes after.