[!390] Fix invalid iterator derefence bug in BCRSPattern

Merge branch 'fix/bcrspattern-invalid-iterator-dereference' into 'master'

Always make sure an iterator is valid after incrementing it...

See merge request pdelab/dune-pdelab!390

(cherry picked from commit 02f35f95)

8ec18c54 Fix invalid iterator derefence bug in BCRSPattern