Add_row fails for empty masked table

```
In [1]: from astropy.table import Table

In [2]: t = Table(names=('a'), dtypes=('f'))

In [3]: t
Out[3]: 
<Table rows=0 names=('a')>
array([], 
      dtype=[('a', '<f4')])

In [4]: t.add_row([1])

In [5]: t
Out[5]: 
<Table rows=1 names=('a')>
array([(1.0,)], 
      dtype=[('a', '<f4')])

In [6]: tm = Table(names=('a'), dtypes=('f'), masked=True)

In [7]: tm.add_row([1])
ERROR: TypeError: Empty data-type [numpy.core.fromnumeric]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-7-bd2d0c983c23> in <module>()
----> 1 tm.add_row([1])

/data/baffin/tom/git/astropy/astropy/table/table.pyc in add_row(self, vals, mask)
   1684 
   1685         if self.masked:
-> 1686             self._data = ma.resize(self._data, (newlen,))
   1687         else:
   1688             self._data.resize((newlen,), refcheck=False)

/data/cosmos2/ska/arch/x86_64-linux_CentOS-5/lib/python2.7/site-packages/numpy/ma/core.pyc in resize(x, new_shape)
   6460     m = getmask(x)
   6461     if m is not nomask:
-> 6462         m = np.resize(m, new_shape)
   6463     result = np.resize(x, new_shape).view(get_masked_subclass(x))
   6464     if result.ndim:

/data/cosmos2/ska/arch/x86_64-linux_CentOS-5/lib/python2.7/site-packages/numpy/core/fromnumeric.pyc in resize(a, new_shape)
    850     a = ravel(a)
    851     Na = len(a)
--> 852     if not Na: return mu.zeros(new_shape, a.dtype.char)
    853     total_size = um.multiply.reduce(new_shape)
    854     n_copies = int(total_size / Na)

TypeError: Empty data-type
```