Fix bi_valid handling in deflate_quick()

Fix bi_valid handling in deflate_quick()

The attached test started failing after commit ad89d5131b29 ("Don't
write end of last block when returning finish_started."): either with
"bi_buf not flushed" message in debug builds, or by producing corrupted
output in release builds.

The problem is that we must not return finish_started when bi_buf is
not empty, because the bits there will be lost. Fix by checking that
bi_valid is not 0.