Fix block_open handling in deflate_quick()

Fix block_open handling in deflate_quick()

The attached test fails with "inflate() failed", because the deflate
stream that it produces ends up being corrupted. Bisect points to the
commit e7bb6db09a18 ("Replace hash_bits, hash_size and hash_mask with
defines."), but it's most likely a coincidence.

In any case, the reason is that if we happen to simultaneously exhaust
all the buffers (in, out and bi), we return finish_started without
writing the end of block symbol, which will never happen afterwards.

Fix by adding another check to the tricky condition: if we are in the
middle of a block, return need_more instead of finish_started.