Fixed register_window_class

First, try to unregister previously registered class.

src/osd/modules/debugger/win/debugwininfo.c

@@ -591,6 +591,8 @@ void debugwin_info::register_window_class()
 		wc.style            = 0;
 		wc.cbClsExtra       = 0;
 		wc.cbWndExtra       = 0;
+		
+		UnregisterClass(wc.lpszClassName, wc.hInstance);
 
 		// register the class; fail if we can't
 		if (!RegisterClass(&wc))