best_access_path() is called from two optimization phases:

1. Plan choice phase, in choose_plan(). Here, the join prefix being
   considered is in join->positions[]

2. Plan refinement stage, in fix_semijoin_strategies_for_picked_join_order
   Here, the join prefix is in join->best_positions[]

It used to access join->positions[] from stage #2. This didnt cause any
valgrind or asan failures (as join->positions[] has been written-to before)
but the effect was similar to that of reading the random data:
The join prefix we've picked (in join->best_positions) could have
nothing in common with the join prefix that was last to be considered
(in join->positions).