Bluetooth: audio_ ascs: Fix possible dereference of free'd ASE

This fixes possible dereference of free'd bt_ascs_ase object.
The ASE without ISO connection that goes to Releasing state will be
automously transitioned to Idle state. Thus, the `ase` object might not
be vaild anymore.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>