BUG: lexsort segfault
Found while fuzz-testing the numpy/pandas interface the other day:

```
>>> import numpy as np, pandas as pd
>>> pd.__version__
'0.13.0rc1-47-g9ac008c'
>>> df = pd.DataFrame(range(5))
>>> np.lexsort(df.values)
array([0])
>>> np.lexsort(df)
Segmentation fault
```
