usnic: explicitly handle case when both endpoints are NULL

If we don't explicitly declare that (a == NULL && b == NULL) is
equivalent to qsort, we could end up with wonky sorting order.  I.e.,
it's *possible* that some NULLs could end up in the middle of the
array.

Regardless of whether it will ever happen in practice, it makes the
code more clear to also handle the "both are NULL" case.

Also fix the 2-spacing indents.

Reviewed by Dave Goodell.

cmr=v1.8.2:reviewer=ompi-rm1.8