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.
(cherry picked from commit open-mpi/hwloc@45c94c336d)
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>