fix panic when Unfold sink return an error (#2686)

- fix issue #2600. When an Unfold sink return an error
  it is left in an invalid state. Calling after that flush/close
  cause the sink to panic due to re-calling a future already completed.

  This patch aims to leave the sink in a valid state and allow calling
  flush/close without causing a panic.

Author: erebe

futures-util/src/sink/unfold.rs

@@ -73,7 +73,10 @@ where
                     this.state.set(UnfoldState::Value { value: state });
                     Ok(())
                 }
-                Err(err) => Err(err),
+                Err(err) => {
+                    this.state.set(UnfoldState::Empty);
+                    Err(err)
+                }
             }
         } else {
             Ok(())