Fix possible segfault when linear solver fails.

Calling solve() after failed compute() can lead to a segfault with Eigen's SparseLU solver.

This MR changes a documentation script, too, such that /* and */ can be used as comments in exprtk expressions. Using XML do not work there under all circumstances, because OGS's XML parser eats whitespace around <!-- -->.
See e.g. https://ogs.ogs.xyz/-/ogs/-/jobs/584555/artifacts/build/docs/d7/d13/ogs_ctest_prj__Mechanics__Linear__cube_1e0_fail_lin_solver__xml.html (might outdate, soon)

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?