[3.13] gh-129732: Fix race on `shared->array` in qsbr code under free-threading (gh-129738) (gh-129747)

The read of `shared->array` should happen under the lock to avoid a race.
(cherry picked from commit b4ff8b22b3066b814c3758f87eaddfa923e657ed)

Co-authored-by: Peter Hawkins <phawkins@google.com>