Avoid abort in dev-build

If not enforcing QOS, it's possible to submit a job without a qos. If
submitting such a job to multiple partitions where at least one has a
qos, slurmctld would abort in a development build. A non-development
build didn't segfault only because _find_qos_part doesn't dereference
the NULL pointer. Prevent the abort.

Bug 7171