Fix heap corruption when calling ub_ctx_delete in Windows

[lnzhu]

ub_ctx_delete in libunbound causes heap corruption exception in Windows OS. This issue is only for Windows and discovered when using the lastest libunbound Windows library published here.

• ub_ctx_delete stops worker thread by ub_stop_bg.
• When the workder thread exits, tube_remove_bg_listen is called for ctx->qq_pipe and the memory of tube->ev_listen is released in ub_winsock_unregister_wsaevent but tube->ev_listen is never set to NULL.
• After worker thread stops, tube_delete is called for tx->qq_pipe in ub_ctx_delete.
• tube_delete calls tube_remove_bg_listen again and it tried to free tube->ev_listen again which leads to heap corruption.
• I tested multiple older versions (back to 2019) of libunbound Windows library and they all have the same issue.
• A cleaner way to fix this issue might be setting NULL inside ub_winsock_unregister_wsaevent but it will need to change ub_winsock_unregister_wsaevent API and pass ub_event** instead, which will bring additional changes in different places.