event/Loop: fix assertion failure due to wrong "busy" value

If an InjectEvent callback schedules a timer, the loop will restart,
but the "busy" flag is still false.  The fix is to move the "again"
check before the "busy" setting.

src/event/Loop.cxx

@@ -326,15 +326,19 @@ EventLoop::Run() noexcept
 		{
 			const std::lock_guard<Mutex> lock(mutex);
 			HandleInject();
+#endif
+
+			if (again)
+				/* re-evaluate timers because one of
+				   the DeferEvents may have added a
+				   new timeout */
+				continue;
+
+#ifdef HAVE_THREADED_EVENT_LOOP
 			busy = false;
 		}
 #endif
 
-		if (again)
-			/* re-evaluate timers because one of the
-			   DeferEvents may have added a new timeout */
-			continue;
-
 		/* wait for new event */
 
 		Wait(timeout);