MEDIUM: muxes: Be prepared to don't own connection during the release

MEDIUM: muxes: Be prepared to don't own connection during the release

This happens during mux upgrades. In such case, when the destroy() callback is
called, the connection points to a different mux's context than the one passed
to the callback. It means the connection is owned by another mux. The old mux is
then released but the connection is not closed.