MINOR: muxes: Don't expect to have a mux without connection in destroy callback

MINOR: muxes: Don't expect to have a mux without connection in destroy callback

Once a mux initialized, the underlying connection alwaus exists from its
point of view and it is never removed until the mux is released. It may be
owned by another mux during an upgrade. But the pointer remains set. Thus
there is no reason to test it in the destroy callback function.

This patch should fix the issue #1652.