[!235] Avoid clang7 self-assignment warning for isNaN /= isNaN.

Merge branch 'bugfix/self-assignment' into 'master'

ref:core/dune-istl Indeed, isNaN could be something complicated, although
normally it just should be double.

See also merge request [dune-common!573] and [dune-common!578]

See merge request [!235]

  [dune-common!573]: gitlab.dune-project.org/Nonedune-common/merge_requests/573
  [dune-common!578]: gitlab.dune-project.org/Nonedune-common/merge_requests/578
  [!235]: gitlab.dune-project.org/core/dune-istl/merge_requests/235