event/Loop: make IsInsideOrNull() available in the NDEBUG build

Fixes build breakage by commit 4e5271fcdf7; and this method does make
sense in non-debug builds.

src/event/Loop.cxx

@@ -222,8 +222,9 @@ EventLoop::Run()
 #ifndef NDEBUG
 	assert(busy);
 	assert(thread.IsInside());
-	thread = ThreadId::Null();
 #endif
+
+	thread = ThreadId::Null();
 }
 
 void

src/event/Loop.hxx

@@ -212,12 +212,16 @@ public:
 	}
 #endif
 
-#ifndef NDEBUG
+	/**
+	 * Like IsInside(), but also returns true if the thread has
+	 * already ended (or was not started yet).  This is useful for
+	 * code which may run during startup or shutdown, when events
+	 * are not yet/anymore handled.
+	 */
 	gcc_pure
 	bool IsInsideOrNull() const {
 		return thread.IsNull() || thread.IsInside();
 	}
-#endif
 };
 
 #endif /* MAIN_NOTIFY_H */