Fix convert from PSparseMatrix to PetscMatrix for MPI backend.

[fverdugo]

No description provided.

[fverdugo]

This fix might be important to you @amartinhuertas.

Specially if you tried to assemble to SparseMatrixCSR{0,PetscScalar,PetscInt} to avoid copies.

The input of convert should be read-only, and we were modifying it here.

I haven't had forces to add a test (it's late here). Could you help with this?

[amartinhuertas]

I haven't had forces to add a test (it's late here). Could you help with this?

Yes. I will add a test.

[fverdugo]

@amartinhuertas I merge this since I need this fix asap. I'll open an issue about the missing test.