Writing subfunction to xml file seg faults

[Marie Rognes]

I get a segmentation fault with the below:

from dolfin import *
mesh = UnitSquare(2, 2)
V = FunctionSpace(mesh, "CG", 1)
W = V*V
vs = Function(W)
(v, s) = vs.split()
file = File("foo.xml")
file << v

This is easily avoidable by doing split(deepcopy=True), but an error message would be significantly better than the seg fault response.

[Garth Wells (garth-wells)]

Probably affects XDMF too. We should add a unit test.