Fix unused warning for un-accessed enum variant field in net-tokio

Author: TheBlueMatt

lightning-net-tokio/src/lib.rs

@@ -208,7 +208,12 @@ impl Connection {
 						break Disconnect::CloseConnection;
 					}
 				},
-				SelectorOutput::B(_) => {},
+				SelectorOutput::B(some) => {
+					// The mpsc Receiver should only return `None` if the write side has been
+					// dropped, but that shouldn't be possible since its referenced by the Self in
+					// `us`.
+					debug_assert!(some.is_some());
+				},
 				SelectorOutput::C(res) => {
 					if res.is_err() { break Disconnect::PeerDisconnected; }
 					match reader.try_read(&mut buf) {