Fix crash when Destroy()-ing a TLW with a non-TLW parent.

Generalize the code in wxTopLevelWindowBase dtor checking for the children of
the TLW being destroyed pending for deletion themselves to work when the child
TLW is an indirect child, i.e. was created with a child window of this TLW as
parent and not this TLW itself.

Closes #15743.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75341 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775