MaskedArray.dot leaks masked values

```
>>> numpy.__version__
'1.9.0'
>>> x = numpy.ma.masked_array([1., 3e10, 3], mask=[0, 1, 0])
>>> x.dot(x)
9e+20
```

Note that this is a regression from oldnumeric:

```
>>> import numpy.oldnumeric.ma
>>> numpy.__version__
'1.8.1'
>>> x = numpy.oldnumeric.ma.masked_array([1., 3e10, 3], mask=[0, 1, 0])
>>> x.dot(x)
array(10.0)
```
