Test galerkintest fails in parallel

Running it on two cores triggers the following assertion (on master and release/2.6):

21: galerkintest: /home/timok/dumux-dune-master/dune-common/dune/common/parallel/indexset.hh:1045: Dune::GlobalLookupIndexSet<I>::GlobalLookupIndexSet(const I&, std::size_t) [with I = Dune::ParallelIndexSet<int, Dune::ParallelLocalIndex<Dune::OwnerOverlapCopyAttributeSet::AttributeSet>, 512>; std::size_t = long unsigned int]: Assertion `pair->local()<size_' failed.
21: [pumbaa:27576] *** Process received signal ***
21: [pumbaa:27576] Signal: Aborted (6)
21: [pumbaa:27576] Signal code:  (-6)

See !295