UI: Fix stats window geometry saving on shutdown

Possible fix for: https://obsproject.com/mantis/view.php?id=923

If you close whole application (not just stats window) it skips the
close event and thus saving of the geometry.  The Qt::WA_DeleteOnClose
was set earlier.

Closes jp9000/obs-studio#984

UI/window-basic-main.cpp

@@ -2943,7 +2943,7 @@ void OBSBasic::CloseDialogs()
 		projector.clear();
 	}
 
-	delete stats;
+	if (!stats.isNull()) stats->close(); //call close to save Stats geometry
 }
 
 void OBSBasic::EnumDialogs()