objectManager: correct invalid index code in onNameVanished

objectManager: correct invalid index code in onNameVanished

The object manager tries to synthesize interface removal
events if the bus name of a remote object drops off the bus.

The code has a bad typo in it, though: it confuses `objectPaths`
(the list of all object paths) and `objectPath` (the object
currently being processed this iteration of the loop).

That leads to a failure to synthesize the interface removal
events, and spew in the log.

This commit corrects the objectPath/objectPaths confusion.