rdf_statistics(graph(N)) incorrect

[wouterbeek]

RDF statistics does not show the correct number of graphs in case a graph is unloaded.

Reproduce:

?- use_module(library(semweb/rdf_db)).
true.
?- rdf_create_graph(test).
true.
?- rdf_unload_graph(test).
true.
?- rdf_statistics(graphs(N)).
N = 1.
?- rdf_graph(G).
false.