input/last: call Close() in Open()

Prevents a possible bug which occurs when the caller-provided open()
function throws; then the "uri" field is never set.

src/input/LastInputStream.hxx

@@ -64,8 +64,7 @@ public:
 			return is.get();
 		}
 
-		is.reset();
-		close_timer.Cancel();
+		Close();
 
 		is = open(new_uri, mutex);
 		uri = std::forward<U>(new_uri);