event/SocketEvent: add assert()

src/event/SocketEvent.cxx

@@ -60,6 +60,8 @@ SocketEvent::Abandon() noexcept
 bool
 SocketEvent::Schedule(unsigned flags) noexcept
 {
+	assert((flags & IMPLICIT_FLAGS) == 0);
+
 	if (flags == GetScheduledFlags())
 		return true;