density plot on zero series throws exception
``` python
    pd.Series(np.zeros(4)).to_frame().plot(kind='kde')
```

Executing the above line throws the below exception. 

```
LinAlgError                               Traceback (most recent call last)
<ipython-input-118-eac1a90078a2> in <module>()
----> 1 pd.Series(np.zeros(4)).to_frame().plot(kind='kde')
```

pandas version 0.16.2
