Avoid creating empty bitmap in MSW wxNotebook::OnPaint()

It can apparently happen that the window has 0 size and in this creating
the bitmap is useless and just triggers an assert.

Closes #18879.

src/msw/notebook.cpp

@@ -787,11 +787,13 @@ void wxNotebook::OnEraseBackground(wxEraseEvent& WXUNUSED(event))
 void wxNotebook::OnPaint(wxPaintEvent& WXUNUSED(event))
 {
     wxPaintDC dc(this);
-    wxMemoryDC memdc;
     RECT rc;
     ::GetClientRect(GetHwnd(), &rc);
+    if ( !rc.right || !rc.bottom )
+        return;
+
     wxBitmap bmp(rc.right, rc.bottom);
-    memdc.SelectObject(bmp);
+    wxMemoryDC memdc(bmp);
 
     const wxLayoutDirection dir = dc.GetLayoutDirection();
     memdc.SetLayoutDirection(dir);