GetImageListBitmaps() -- don't modify if invalid

If the bitmap is empty or invalid, there is no reason to attempt to add a mask,
alpha channel, etc.

src/msw/imaglist.cpp

@@ -167,6 +167,12 @@ void
 wxImageList::GetImageListBitmaps(wxMSWBitmaps& bitmaps,
                                  const wxBitmap& bitmap, const wxBitmap& mask)
 {
+    if (!bitmap.IsOk())
+    {
+        // We can't do anything with an invalid bitmap.
+        return;
+    }
+
     // This can be overwritten below if we need to modify the bitmap, but it
     // doesn't cost anything to initialize the bitmap with this HBITMAP.
     bitmaps.hbmp = GetHbitmapOf(bitmap);