Don't assert when destroying a never created window in wxGTK.

Don't assert when destroying a never created window in wxGTK.

The other ports don't assert if a default-constructed wxWindow object for
which Create() had never been called is Destroy()d and wxGTK shouldn't do this
neither.

The new behaviour is more logical but also fixes a problem with an assert in
wxOwnerDrawnComboBox that can currently be seen in wxGTK unit tests.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67539 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775