vtk output fails for P0 functions

vtk output fails if function space of plotted function does not match coordinate function space.

Example code:
```python
from firedrake import *
m = UnitSquareMesh(2, 2)
output = File('volume.vtu')
output << project(m._coordinate_field, VectorFunctionSpace(m, 'DG', 1))
```

output:
```
eikemueller@dyn898-216 $ python AreaOfTriangle.py 
[ 0.98117818]
Traceback (most recent call last):
  File "AreaOfTriangle.py", line 55, in <module>
    output << cell_volume
  File "/Users/eikemueller/PostDocBath/EllipticSolvers/firedrake/python/firedrake/io.py", line 95, in __lshift__
    self._file << data
  File "/Users/eikemueller/PostDocBath/EllipticSolvers/firedrake/python/firedrake/io.py", line 192, in __lshift__
    if not self._warnings[0]:
TypeError: 'NoneType' object has no attribute '__getitem__'
```