len(table) fails when self._data is numpy.ma.core.mvoid (and scalar?)

This error appears when trying to get the length of a table that is of type `mvoid`:

```
    def __len__(self):
       return len(self._data)
       TypeError: len() of unsized object

/home/travis/virtualenv/python2.7/src/astropy/astropy/table/table.py:689: TypeError
```
From the astroquery .travis run, in `test_vizier`:
https://travis-ci.org/keflavich/astrodata/jobs/4224400

Using pdb, I examined more closely:
```
ipdb> self._data
(2, 60.27763, -16.110863, 60.27763, -16.110863, '04010663-1606391', 12.175999641418457)
ipdb> self._data.__len__
<method-wrapper '__len__' of mvoid object at 0x104e25c80>
ipdb> len(self._data)
*** TypeError: len() of unsized object
ipdb> self._data.dtype
dtype([('_q', '<i4'), ('_RAJ2000', '<f8'), ('_DEJ2000', '<f8'), ('RAJ2000', '<f8'), ('DEJ2000', '<f8'), ('2MASS', '|S17'), ('Kmag', '<f4')])
ipdb> self._data.shape
()
```

So it looks like this is an error because it's a length-zero recordarray?