Series.corr failure
The zero-length check in nancorr is in the wrong place, see below:

```
from pandas import Series
import numpy as np
s1 = Series( index = range(3) , data = [np.NaN , np.NaN , np.NaN] )
s2 = Series( index = range(3) , data = [np.NaN , np.NaN , np.NaN] )
s2.corr(s1)
```
