[MatL/S] Fix a bug in MCASAnisotropic in 2D case.

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

The silently running, but still incorrect project file is Mechanics/MohrCoulombAbboSloanAnisotropic/triax_aniso_1e0_47.prj. The error is observable with valgrind or address sanitizer.