[PATCH 3/7 V3] idr: fix NULL pointer dereference when ida_remove(unallocated_id)

If the ida has at least one existing id, and when an unallocated ID which meets a certain condition is passed to the ida_remove(), the system will crash because it hits NULL pointer dereference. The condition is that the unallocated ID shares the same lowest idr layer with the existing ID, but

[PATCH 3/7 V3] idr: fix NULL pointer dereference when ida_remove(unallocated_id)

If the ida has at least one existing id, and when an unallocated ID which meets a certain condition is passed to the ida_remove(), the system will crash because it hits NULL pointer dereference. The condition is that the unallocated ID shares the same lowest idr layer with the existing ID, but