Don't use wxToolBarToolBase::SetNormalBitmap() in wxToolbook.

This doesn't work, use wxToolBar::SetToolNormalBitmap() which does.

As a side effect, don't silently ignore invalid page indices in
wxToolbook::SetPageImage() any more, they should result in an assert, which is
now going to happen inside wxToolBar::SetToolNormalBitmap() if this method is
called with an invalid index.

See #16985.

src/generic/toolbkg.cpp

@@ -167,16 +167,9 @@ bool wxToolbook::SetPageImage(size_t n, int imageId)
     if (!GetImageList())
         return false;
 
-    wxToolBarToolBase* tool = GetToolBar()->FindById(n + 1);
-    if (tool)
-    {
-        // Find the image list index for this tool
-        wxBitmap bitmap = GetImageList()->GetBitmap(imageId);
-        tool->SetNormalBitmap(bitmap);
-        return true;
-    }
-    else
-        return false;
+    GetToolBar()->SetToolNormalBitmap(n + 1, GetImageList()->GetBitmap(imageId));
+
+    return true;
 }
 
 // ----------------------------------------------------------------------------