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.

[gthess]

Thanks, this looks good! But I don't think the #if defined is required. I see why you have it there, but always setting tube->ev_listen = NULL seems fine regardless if USE_MINI_EVENT is defined or not. Unless I miss something.

[lnzhu]

Thanks for reviewing! Yes, #if defined(USE_MINI_EVENT) is not strictly needed in current code. But tube->ev_listen is deleted in ub_winsock_unregister_wsaevent only if USE_MINI_EVENT is defined. I think it might be safer and future-proof to have the check before calling ub_winsock_unregister_wsaevent.

[gthess]

It is indeed only freed when USE_MINI_EVENT is defined. For the other cases (when not defined, or when ub_event_pluggable.c is used) setting tube->ev_listen = NULL always after the ub_winsock_unregister_wsaevent() call conveys the correct intent that this listener event is no more.

I also prefer that all the USE_MINI_EVENT #ifdefs are handled in ub_event(_pluggable).c and not exposed in tube.c.

[lnzhu]

removed #if defined in the last commit