Avoid asserts when drawing zero-size wxStaticBox in wxMSW

If a wxStaticBox is rendered, but has zero size in either dimension we
generate assert failures when creating the bitmap. Check for these conditions
and just do not render if this is the case.

Closes #17288.

src/msw/statbox.cpp

@@ -507,6 +507,11 @@ void wxStaticBox::OnPaint(wxPaintEvent& WXUNUSED(event))
     ::GetClientRect(GetHwnd(), &rc);
     wxPaintDC dc(this);
 
+	// No need to do anything if the client rectangle is empty and, worse,
+    // doing it would result in an assert when creating the bitmap below.
+	if ( !rc.right || !rc.bottom )
+		return;
+
     // draw the entire box in a memory DC
     wxMemoryDC memdc(&dc);
     wxBitmap bitmap(rc.right, rc.bottom);