Don't leak memory when receiving TaskbarButtonCreated messages.

It is possible to receive more than one TaskbarButtonCreated message for the
same window, so at the very least don't leak memory if this happens.

We really ought to notify the application to allow it to reinitialize the
taskbar button in this case though.

See #16566.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77907 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775