Orders API: Fix race condition in voucher redemption (Z#23230391) (#6067)

The old code relied on the `Voucher.redeemed` value obtained *before*
the lock was taken, not afterwards.

The change in services/orders.py is functionally pointless, but it makes
the pattern of "fill availability only after lock" clearer and might
avoid introducing similar bugs in the future.

src/pretix/base/services/orders.py

@@ -727,8 +727,6 @@ def _check_positions(event: Event, now_dt: datetime, time_machine_now_dt: dateti
     _check_date(event, time_machine_now_dt)
 
     products_seen = Counter()
-    q_avail = Counter()
-    v_avail = Counter()
     v_usages = Counter()
     v_budget = {}
     deleted_positions = set()
@@ -793,6 +791,9 @@ def _check_positions(event: Event, now_dt: datetime, time_machine_now_dt: dateti
                 shared_lock_objects=[event]
             )
 
+    q_avail = Counter()
+    v_avail = Counter()
+
     # Check maximum order size
     limit = min(int(event.settings.max_items_per_order), settings.PRETIX_MAX_ORDER_SIZE)
     if sum(1 for cp in sorted_positions if not cp.addon_to) > limit: