UnboundLocalError when doing join().join_from()
Nevermind, I made a mistake in my code and `ModelAModelB` was not `ModelAModelB`.
I have a query that looks like this:

```
ModelAModelB
.select(ModelA, ModelB)
.join(ModelC, on=(ModelC.some_field == ModelAModelB.some_field))
.join_from(ModelAModelB, ModelA) # Exception occurs on this line
.join_from(ModelAModelB, ModelB)
```

For some reason, today I got an error:

```

2019-02-20T11:33:07.005118570Z   File "/usr/local/lib/python3.7/site-packages/peewee.py", line 6061, in join_from
2019-02-20T11:33:07.005130799Z     return self.join(dest, join_type, on, src, attr)                            
2019-02-20T11:33:07.005142575Z   File "/usr/local/lib/python3.7/site-packages/peewee.py", line 606, in inner             
2019-02-20T11:33:07.005154599Z     method(clone, *args, **kwargs)                                                        
2019-02-20T11:33:07.005166359Z   File "/usr/local/lib/python3.7/site-packages/peewee.py", line 6050, in join             
2019-02-20T11:33:07.005178361Z     on, attr, constructor = self._normalize_join(src, dest, on, attr)
2019-02-20T11:33:07.005190257Z   File "/usr/local/lib/python3.7/site-packages/peewee.py", line 6008, in _normalize_join  
2019-02-20T11:33:07.005202328Z     return (on, attr, constructor)                                                        
2019-02-20T11:33:07.005214005Z UnboundLocalError: local variable 'constructor' referenced before assignment
```

Is there something I'm doing wrong? I thought about changing `join` to `join_from(ModelAModelB, ModelC)`, but I'm not sure that's the reason of this error.