Fix call to draw when no notifications are available

Draw should not be called when there are no notifications in the queue.
This for unknown reasons causes a crash the next time we attempt to get
the currently active window with a BadValue error from
XGetWindowProperty.

Fixes #630.

src/draw.c

@@ -611,6 +611,7 @@ static void calc_window_pos(int width, int height, int *ret_x, int *ret_y)
 
 void draw(void)
 {
+        assert(queues_length_displayed() > 0);
 
         GSList *layouts = create_layouts(x_win_get_context(win));
 

src/x11/x.c

@@ -341,7 +341,10 @@ gboolean x_mainloop_fd_dispatch(GSource *source, GSourceFunc callback, gpointer
                                 LOG_D("XEvent: processing PropertyNotify for Resource manager");
                                 XRM_update_db();
                                 screen_dpi_xft_cache_purge();
-                                draw();
+
+                                if (win->visible) {
+                                        draw();
+                                }
                                 break;
                         }
                         /* Explicitly fallthrough. Other PropertyNotify events, e.g. catching