Mem leak when using callables that reference an instance

Without this patch this simple test will blow your memory:
```python
import time
import hiredis

class X:
    def __init__(self):
        self.data = 'a'*1024*1024
    def error(self):
        return Exception

while True:
    x = X()
    z = hiredis.Reader(replyError=x.error)
    time.sleep(1)
```

This patch fixes a memory leak when hiredis is used in conjunction with redis-py