Don't crash in wxAuiToolBar::GetToolBarFits().

The code could crash if m_overflowVisible was true but m_overflowSizerItem was
NULL. Ideally this shouldn't happen in the first place, but it did, so at
least fix the crash for now -- and hopefully this code will be refactored to
make it more clear later.

Closes #15747.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75400 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775