Another try at fixing the ps segfault resulting from /proc entries vanishing

Another try at fixing the ps segfault resulting from /proc entries vanishing
out from under us due to asyncronous process exit.

The directory we're traversing vanishing can result in DIRTREE_ABORTVAL
being returned, which we turn into a NUL entry, but then we were trying to
look at that null entry's children. Oops.