mux: don't panic the server if the client disconnects

I saw that the server exited with a panic here in the case that the
connection was torn down and left the channel broken.  We don't need
to terminate in this case and can simply ignore the error.

src/server/listener/sessionhandler.rs

@@ -185,9 +185,7 @@ impl SessionHandler {
                 }),
             };
             log::trace!("{} processing time {:?}", serial, start.elapsed());
-            sender
-                .send(DecodedPdu { pdu, serial })
-                .expect("failed to send DecodedPdu to sender")
+            sender.send(DecodedPdu { pdu, serial }).ok();
         };
 
         fn catch<F, SND>(f: F, send_response: SND)