circpad_setup_machine_on_circ(): exit early on error.
This function does a nonfatal assertion to make sure that a machine is not registered twice, but Tobias Pulls found a case where it happens. Instead, make the function exit early so that it doesn't cause a remotely triggered memory leak. Fixes bug 33619; bugfix on 0.4.0.1-alpha. This is also tracked as TROVE-2020-004.