TMM ports cannot exist without a backend.
This fixes crashes where the PortEngine is re-initialized
(backend change):
Previously, TMM ports were only dropped (and recreated)
in TMM::restart() called from ARDOUR::init_post_engine().
When starting a new backend the old one is already destroyed,
but TM still held reference to Ports owned by that backend.
Calling the port's d'tor caused a memory corruption trying to
acquire the backend's port_callback_mutex:
e.g. When creating new session with a different backend
from a running instance, or unit-test after call to
AudioEngine::destroy(): https://pastebin.com/4D6pLA5s
32a0c6a3ff