`ThumbnailCtrl` on Phoenix: fix a ZeroDivisionError when the ThumbnailCtrl is minimized inside a AUI pane.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxPython/Phoenix/trunk@75890 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775