Segfault observed in SystemAssembler

Just dumping this here now for the record:

fem/test_system_assembler.py::test_facet_assembly 
Thread 1 "python" received signal SIGSEGV, Segmentation fault.
dolfin::SystemAssembler::compute_interior_facet_tensor (ufc=..., ufc_cell=..., 
    coordinate_dofs=..., tensor_required_cell=..., tensor_required_facet=..., 
    cell=..., local_facet=..., facet_owner=true, cell_integrals=..., 
    interior_facet_integrals=..., matrix_size=..., vector_size=0, 
    compute_cell_tensor=...) at ../../dolfin/fem/SystemAssembler.cpp:955
---Type <return> to continue, or q <return> to quit---
955                             interior_facet_integrals[form]->enabled_coefficients());
A debugging session is active.

        Inferior 1 [process 6606] will be killed.

Quit anyway? (y or n) n
Not confirmed.
(gdb) where
#0  dolfin::SystemAssembler::compute_interior_facet_tensor (ufc=..., 
    ufc_cell=..., coordinate_dofs=..., tensor_required_cell=..., 
    tensor_required_facet=..., cell=..., local_facet=..., facet_owner=true, 
    cell_integrals=..., interior_facet_integrals=..., matrix_size=..., 
    vector_size=0, compute_cell_tensor=...)
    at ../../dolfin/fem/SystemAssembler.cpp:955
#1  0x00007fffdc7fbf20 in dolfin::SystemAssembler::facet_wise_assembly (
    tensors=..., ufc=..., data=..., 
    boundary_values=std::vector of length 1, capacity 1 = {...}, cell_domains=
    std::shared_ptr (empty) 0x0, 
    exterior_facet_domains=std::shared_ptr (empty) 0x0, 
    interior_facet_domains=std::shared_ptr (empty) 0x0)
    at ../../dolfin/fem/SystemAssembler.cpp:737
#2  0x00007fffdc7f7f3a in dolfin::SystemAssembler::assemble (this=0x23560e0, A=
    0x23b3b60, b=0x0, x0=0x0) at ../../dolfin/fem/SystemAssembler.cpp:290
#3  0x00007fffdc7f5afa in dolfin::SystemAssembler::assemble (this=0x23560e0, A=
    ...) at ../../dolfin/fem/SystemAssembler.cpp:68
#4  0x00007fffbf16f690 in _wrap_SystemAssembler_assemble__SWIG_1 (nobjs=2, 
    swig_obj=0x7fffffff6850) at modulePYTHON_wrap.cxx:27200
#5  0x00007fffbf170367 in _wrap_SystemAssembler_assemble (self=0x7fffbf7abd18, 
    args=0x7fffee406cc8) at modulePYTHON_wrap.cxx:27451
#6  0x00000000004e172f in PyCFunction_Call ()

The problem seems to be that interior_facet_integrals[1] on the last line here is nullptr:

  // Compute facet contribution to tensor, if required
  // Loop over lhs and then rhs facet contributions
  for (std::size_t form = 0; form < 2; ++form)
  {
    // Compute interior facet integral
    if (tensor_required_facet[form] && facet_owner)
    {
      // Update to current pair of cells
      ufc[form]->update(cell[0], coordinate_dofs[0], ufc_cell[0],
                        cell[1], coordinate_dofs[1], ufc_cell[1],
                        interior_facet_integrals[form]->enabled_coefficients());

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