libtracker-data: Unset graph after Modify clause

If the Modify clause sets a graph via 'WITH' the graph token remains but the solution underneath gets freed. That means the graph token's literal is kept "set" but containing garbage, and may wreak havoc if there are more Update clauses in the same update string that don't specify a graph, as that garbage value will be used. This may lead to warnings, crashes, and bogus graph databases being created. Eek.

The 'WITH' graph is not meant to be carried over, so ensure it gets unset after dealing with the Modify clause.