Functions on SubDomains throw an error with MPI-full

The following MFE fails with `DEVITO_MPI='full'`, even on a single rank.

MFE:
```python
from devito import Grid, SubDomain, Function, Eq, Operator


class SD0(SubDomain):
    name = 'sd0'

    def define(self, dimensions):
        return {d: ('middle', 2, 2) for d in dimensions}


grid = Grid(shape=(11,))
sd0 = SD0(grid=grid)

f = Function(name='f', grid=sd0)
eq = Eq(f, f+1)

Operator(eq)()
```