Restore the old logger in wxLogChain dtor instead of deleting it.

Restore the old logger in wxLogChain dtor instead of deleting it.

wxLogChain was leaving the global log target pointing to a deleted object,
resulting in crashes when using wxLogWindow without any explicit
SetActiveTarget() calls.

Restore the original logger as the active target in wxLogChain dtor to ensure
that the active log target remains valid.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65956 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775