optimization_tao raises PETSc 83 error

[Simon Funke]

The following errror is raised when running tests_dolfin/optimization_tao:

PetscDolfinErrorHandler: line '2987', function 'MatGetDiagonal_Python', file 'src/libpetsc4py/libpetsc4py.c',
                       : error code '83' (  ), message follows:
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method getDiagonal()
------------------------------------------------------------------------------
PetscDolfinErrorHandler: line '4465', function 'MatGetDiagonal', file '/tmp/petsc-src/src/mat/interface/matrix.c',
                       : error code '83' (  ), message follows:
------------------------------------------------------------------------------

------------------------------------------------------------------------------
PetscDolfinErrorHandler: line '352', function 'TaoSolve_NLS', file '/tmp/petsc-src/src/tao/unconstrained/impls/nls/nls.c',
                       : error code '83' (  ), message follows:
------------------------------------------------------------------------------

------------------------------------------------------------------------------
PetscDolfinErrorHandler: line '206', function 'TaoSolve', file '/tmp/petsc-src/src/tao/interface/taosolver.c',
                       : error code '83' (  ), message follows:
------------------------------------------------------------------------------

------------------------------------------------------------------------------
Traceback (most recent call last):
  File "optimization_tao.py", line 63, in <module>
    m_opt = solver.solve()
  File "/home/fenics/shared/dolfin_adjoint/misc.py", line 33, in func_wrapper
    res = func(*args, **kwargs)
  File "/home/fenics/shared/dolfin_adjoint/optimization/tao_solver.py", line 408, in solve
    self.tao.solve()
  File "PETSc/TAO.pyx", line 437, in petsc4py.PETSc.TAO.solve
petsc4py.PETSc.Error: error code 83
Aborted (core dumped)