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