If MT brotli compression/decompression stops unexpectedly or after canceling, for instance in error case by write, if it was cut or by partial extraction, they may leak:

• because of return that bypass cleanup (affected only extract with -mmt1)
• because some entries may still remain in writelist_done or writelist_busy lists and they, in contrast to writelist_free list, will not be freed (so leaking with wl but also large buffer blocks wl->out.buf);

This fix removes possible memory leakage in such cases.

In addition this PR has a small code deduplication (in brotli-mt_decompress.c:366-371).