Fix T43090: Cycles + Freestyle + border render = black render.

The reported problem was due to a special case where there are no strokes
to be rendered.  Since rendering an empty scene is a waste of time, the issue
was addressed here by skipping the stroke rendering process entirely.

source/blender/freestyle/intern/application/Controller.cpp

@@ -869,7 +869,7 @@ void Controller::DrawStrokes()
 	real d = _Chrono.stop();
 	if (G.debug & G_DEBUG_FREESTYLE) {
 		cout << "Strokes generation  : " << d << endl;
-		cout << "Stroke count  : " << _Canvas->stroke_count << endl;
+		cout << "Stroke count  : " << _Canvas->getStrokeCount() << endl;
 	}
 	resetModified();
 	DeleteViewMap();