Fix bug in TrilinosVector::sadd with MPI

As pointed out by Praveen C on the mailing list, sadd() of the Trilinos vector did
not work properly when combining ghosted with non-ghosted vectors. The test supposed
to check this used two vectors with the same entries, which did not detect that the
wrong vector was scaled.
Merge pull request #174 from kronbichler/master

Fix bug in TrilinosVector::sadd with MPI