Bluetooth: Controller: Fix assertion in scan aux release

Fix assertion under race conditions where LLL scheduling
did not have an auxiliary context assigned but when node rx
is released an auxiliary context is picked from the scan
context that has no parent assigned yet due to a previously
assigned auxiliary context to the scan context was not
cleared.

Relates to commit 49642efa ("Bluetooth: Controller: Fix
regression in scan aux release").

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>