planner: Correct crash in virtual doc iteration

There was an issue with iterating over virtual docs when planning
an expression that had `with` keywords. The iterator would potentially
be called multiple times, so the "with" values needed to be pushed
and popped accordingly. This essentially copies the pattern used in
the topdown eval flow.

Fixes: #2601
Signed-off-by: Patrick East <east.patrick@gmail.com>