debug: fix an overzealous assertion about the parent cpuset vs its children

When I/O are attached under a PU, removing the children's cpusets from
the parent cpuset doesn't give 0, it gives the PU cpuset.
The assertion fails on single-pu machines with I/O when --merge is given,
only one PU remains with I/O under it.

But if we insert Misc by cpuset under PU, it gives 0 as expected.

Fix the assertion accordingly.

Thanks to Thomas Van Doren for reporting the issue.