BUG: Segfault on isnull(MultiIndex)
```
In [1]: from pandas import *

In [2]: idx = MultiIndex.from_tuples([(0, 'a'), (1, 'b'), (2, 'c')])

In [3]: pd.isnull(idx)
Segmentation fault: 11
(pandas-dev)dhcp80fff527:pandas tom$ 
```

This is on latest master (`'0.12.0-757-g69b67f3'`). Numpy 1.7.1.
