popsbr.c: Alter traverse() to check action callback's result.

popsbr.c: Alter traverse() to check action callback's result.

The action callback returned zero, with a 1999 comment asking if it was
checked.  traverse() didn't check it.  Change it so it does, wanting
`OK' if all's well, else returning the non-OK value to traverse's
caller;  though that only checks for NOTOK.  No functional change
intended.