event/Loop: AbandonFD() unlinks the SocketEvent

Fixes use-after-free bugs causing assertion failures at shutdown,
because all "abandoned" SocketEvents are still in the linked list.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/986
Closes https://github.com/MusicPlayerDaemon/MPD/issues/987

src/event/Loop.cxx

@@ -110,6 +110,8 @@ EventLoop::AbandonFD(SocketEvent &event)  noexcept
 #endif
 	assert(event.IsDefined());
 
+	event.unlink();
+
 	return poll_backend.Abandon(event.GetSocket().Get());
 }