“ We are of opinion that the case is moot because, after petitioner's service of his sentence and its expiration, there was no longer a subject matter on which the judgment of this Court could operate. A federal court is without power to decide moot questions or to give advisory opinions which cannot affect the rights of the litigants in the case before it. ”

“ United States v. Alaska S.S.C.o., 253 U.S. 113, 115, 116, 40 S.Ct. 448, 64 L.Ed. 808, and cases cited; United States v. Hamburg-Amerikanische Packet-Fahrt-Actien Gesellschaft, 239 U.S. 466, 475-477, 36 S.Ct. ”