MDEV-31403: Server crashes in st_join_table::choose_best_splitting

The code in choose_best_splitting() assumed that the join prefix is
in join->positions[].

This is not necessarily the case. This function might be called when
the join prefix is in join->best_positions[], too.
Follow the approach from best_access_path(), which calls this function:
pass the current join prefix as an argument,
"const POSITION *join_positions" and use that.

sql/sql_select.cc

@@ -7452,6 +7452,7 @@ best_access_path(JOIN      *join,
   if (s->table->is_splittable())
     spl_plan= s->choose_best_splitting(idx,
                                        remaining_tables,
+                                       join_positions,
                                        &spl_pd_boundary);
 
   Json_writer_array trace_paths(thd, "considered_access_paths");