guard against missing editor element in editor-base-controller.save
guard against missing editor element in editor-base-controller.save

issue #5659
- Fixes occasional issue with `Uncaught TypeError: Cannot read property 'val' of undefined` errors appearing when navigating quickly to/from the editor. I traced the `save` action calls back to the `autoSave` method - it may warrant further investigation to find out why the throttled/debounced calls are sometimes made when the editor element is non-existent.

kevinansfield/Ghost-Admin,airycanon/Ghost-Admin,dbalders/Ghost-Admin,acburdine/Ghost-Admin,acburdine/Ghost-Admin,kevinansfield/Ghost-Admin,airycanon/Ghost-Admin,TryGhost/Ghost-Admin,JohnONolan/Ghost-Admin,TryGhost/Ghost-Admin,dbalders/Ghost-Admin,JohnONolan/Ghost-Admin