Bluetooth: controller: Fix memq_dequeue function

When testing memq implementation used by controller, a
missing check on NULL pointer return could lead to NULL
pointer deferencing.

Current implementation of controller and mayfly do not
by design lead to NULL pointer dereferencing, this fix
is only for correct-ness and complete-ness.

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