mhd_eigen values are unused

[zingale]

In computing the eigensystem of the MHD equations in mhd_eigen.H, we have logic like:

  if (std::abs(cfz-csz) <= 1e-14){
    alf = 1.0_rt;
    als = 0.0_rt;
  }

  if (as - csz < 0.0) {
    alf = 0.0_rt;
  } else {
    alf = std::sqrt((as - csz)/(cfz - csz));
  }

  if (cfz - as < 0.0) {
    als = 0.0_rt;
  } else {
    als = std::sqrt((cfz - as)/(cfz - csz));
  }

So the values set in the first test are never used, since they are overwritten by the else in the next 2 tests.

This was identified by clang-tidy