pxar_backup_stream.rs: limit lock scope to avoid blocking forever

src/client/pxar_backup_stream.rs

@@ -80,10 +80,12 @@ impl Stream for PxarBackupStream {
     type Error = Error;
 
     fn poll(&mut self) -> Poll<Option<Vec<u8>>, Error> {
+        { // limit lock scope
             let error = self.error.lock().unwrap();
             if let Some(ref msg) = *error {
                 return Err(format_err!("{}", msg));
             }
+        }
         self.stream.as_mut().unwrap().poll().map_err(Error::from)
     }
 }