Plan: allocate correct amount of space for temp arrays

Entries in the exec halo of Maps may point to non-exec halo set entries,
we therefore need to allocate total_size rather than exec_size space.
We didn't notice this previously in Firedrake because we only ever had
exec_size being equivalent to total_size.
Merge pull request #384 from OP2/plan-fix

* plan-fix:
  Plan: allocate correct amount of space for temp arrays