Transport Clocksource Message causes crash

I found what seems to be a new bug in transport where Max will crash if the transport is sent a 'clocksource rewire' message when not using the ad_rewire driver.

I realize the most obvious solution is to simply not send this message under these conditions, but I have a 'master clock' patch which I've been using for years across a large number of patches. It uses pattr and sometimes sends this message automatically when loading a patch. Previously the patch would fail gracefully.

Here is an example. Set the driver to something other than ad_rewire (ie built in output) and click the toggle.