BUG: fieldsplit numbering goes wrong with VI

[colinjcotter]

Describe the bug

I can't set up fieldsplits in a VI solver.

Steps to Reproduce

https://gist.github.com/colinjcotter/6237e095d68f2caa1a0cee90ceef2206

Expected behavior

I expect fieldsplits to be correctly set up

Error message

Traceback (most recent call last):
  File "/home/cjc1/repositories/sketchpad/adjustment/minimal.py", line 44, in <module>
    solver.solve(bounds=(lbound, ubound))
  File "petsc4py/PETSc/Log.pyx", line 188, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "petsc4py/PETSc/Log.pyx", line 189, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "/home/cjc1/firedrake-venv/lib/python3.12/site-packages/firedrake/adjoint_utils/variational_solver.py", line 111, in wrapper
    out = solve(self, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^^
  File "/home/cjc1/firedrake-venv/lib/python3.12/site-packages/firedrake/variational_solver.py", line 356, in solve
    self.snes.solve(None, work)
  File "petsc4py/PETSc/SNES.pyx", line 1724, in petsc4py.PETSc.SNES.solve
petsc4py.PETSc.Error: error code 63
[0] SNESSolve() at /home/cjc1/repositories/petsc/src/snes/interface/snes.c:4839
[0] SNESSolve_VINEWTONRSLS() at /home/cjc1/repositories/petsc/src/snes/impls/vi/rs/virs.c:461
[0] KSPSetUp() at /home/cjc1/repositories/petsc/src/ksp/ksp/interface/itfunc.c:415
[0] PCSetUp() at /home/cjc1/repositories/petsc/src/ksp/pc/interface/precon.c:1071
[0] PCSetUp_FieldSplit() at /home/cjc1/repositories/petsc/src/ksp/pc/impls/fieldsplit/fieldsplit.c:695
[0] MatCreateSubMatrix() at /home/cjc1/repositories/petsc/src/mat/interface/matrix.c:8618
[0] MatCreateSubMatrices() at /home/cjc1/repositories/petsc/src/mat/interface/matrix.c:7231
[0] MatCreateSubMatrices_SeqAIJ() at /home/cjc1/repositories/petsc/src/mat/impls/aij/seq/aij.c:2799
[0] MatCreateSubMatrix_SeqAIJ() at /home/cjc1/repositories/petsc/src/mat/impls/aij/seq/aij.c:2516
[0] MatSeqAIJSetPreallocation_SeqAIJ() at /home/cjc1/repositories/petsc/src/mat/impls/aij/seq/aij.c:3983
[0] Argument out of range
[0] nnz cannot be greater than row length: local row 15 value 31 rowlength 21