maxsize=0 in create_pool raise TypeError

if I set `maxsize=0` in `create_pool`, occasionally I got
```sh
Traceback (most recent call last):
File "/xxx/lib/python3.5/site-packages/tornado/web.py", line 1511, in _execute
result = yield result
File "/xxx/lib/python3.5/site-packages/tornado/gen.py", line 1055, in run
value = future.result()
File "/xxx/lib/python3.5/site-packages/tornado/concurrent.py", line 238, in result
raise_exc_info(self._exc_info)
File "", line 4, in raise_exc_info
File "/xxx/lib/python3.5/site-packages/tornado/gen.py", line 307, in wrapper
yielded = next(result)
File "", line 6, in _wrap_awaitable
File "/yyy.py", line 34, in extract_pg
with (await pool.cursor(cursor_factory=cursor)) as cur:
File "/xxx/lib/python3.5/site-packages/aiopg/pool.py", line 263, in cursor
conn = yield from self.acquire()
File "/xxx/lib/python3.5/site-packages/aiopg/utils.py", line 67, in __iter__
resp = yield from self._coro
File "/xxx/lib/python3.5/site-packages/aiopg/pool.py", line 170, in _acquire
yield from self._fill_free_pool(True)
File "/xxx/lib/python3.5/site-packages/aiopg/pool.py", line 212, in _fill_free_pool
if override_min and self.size < self.maxsize:
TypeError: unorderable types: int() < NoneType()
```

using `aiopg=0.13.0`