event/Loop: add thread assert() to AddDefer()

Currently fails in class NfsFileReader due to
MusicPlayerDaemon/MPD#1298

Author: MaxKellermann

src/event/Loop.cxx

@@ -224,6 +224,10 @@ EventLoop::HandleTimers() noexcept
 void
 EventLoop::AddDefer(DeferEvent &e) noexcept
 {
+#ifdef HAVE_THREADED_EVENT_LOOP
+	assert(!IsAlive() || IsInside());
+#endif
+
 	defer.push_back(e);
 
 #ifdef HAVE_THREADED_EVENT_LOOP