event/Loop: remove failing assert()

The `assert(!quit)` can fail if the `EventThread` gets stopped before
it enters `EventLoop::Run()`. There is a similar problem with `alive`,
which gets reset by `EventThread::Stop()`.

If that happens, then `EventLoop::Run()` should return immediately
without handling any events.

src/event/Loop.cxx

@@ -275,10 +275,8 @@ EventLoop::Run() noexcept
 #endif
 
 	assert(IsInside());
-	assert(!quit);
 #ifdef HAVE_THREADED_EVENT_LOOP
-	assert(!quit_injected);
-	assert(alive);
+	assert(alive || quit_injected);
 	assert(busy);
 
 	wake_event.Schedule(SocketEvent::READ);
@@ -303,7 +301,7 @@ EventLoop::Run() noexcept
 
 	FlushClockCaches();
 
-	do {
+	while (!quit) {
 		again = false;
 
 		/* invoke timers */
@@ -365,7 +363,7 @@ EventLoop::Run() noexcept
 
 			socket_event.Dispatch();
 		}
-	} while (!quit);
+	}
 
 #ifdef HAVE_THREADED_EVENT_LOOP
 #ifndef NDEBUG