Fix corruption that rarely occurs in 32-bit mode with wlog=25

Fix corruption that rarely occurs in 32-bit mode with wlog=25

Fix an off-by-one error in the compressor that emits corrupt blocks if:

* Zstd is compiled in 32-bit mode
* The windowLog == 25 exactly
* An offset of 2^25-3, 2^25-2, 2^25-1, or 2^25 is emitted
* The bitstream had 7 bits leftover before writing the offset

This bug has been present since before v1.0, but wasn't able to easily
be triggered, since until somewhat recently zstd wasn't able to find
matches that were within 128KB of the window size.

Add a test case, and fix 2 bugs in `ZSTD_compressSequences()`:
* The `ZSTD_isRLE()` check was incorrect. It wouldn't produce
  corruption, but it could waste CPU and not emit RLE even if the block
  was RLE
* One windowSize was `1 << windowLog`, not `1u << windowLog`

Thanks to @tansy for finding the issue, and giving us a reproducer!

Fixes Issue #3350.