kr_rplan_pop(): avoid reordering kr_rplan::pending

kr_rplan_pop(): avoid reordering kr_rplan::pending

- It's not ideal, but this will need significant overhaul anyway when
  "parallel queries" get implemented.
- I didn't put this into array.h, as we don't seem likely to need it
  anywhere else and implementation in macros would be ugly.
- It's unclear whether this "bug" could cause anything in practice.