Theme key breaks drupal_clone with invalid node id

In doing some large scale content migration, I came across an issue in which the call to _themekey_match_properties from themekey.node.inc would break drupal_clone when called for an invalid node id. This is because the underlying node_load returns an empty object. Wrapping the call to _themekey_match_properties in a check for a valid object fixes this. I checked and didn't see any duplicates for this but apologies if there are.