Store: fix bufio pool handling

pkg/store/bucket.go

@@ -2792,13 +2792,13 @@ func (r *bucketIndexReader) fetchPostings(ctx context.Context, keys []labels.Lab
 		// We assume index does not have any ptrs that has 0 length.
 		length := int64(part.End) - start
 
-		brdr := bufioReaderPool.Get().(*bufio.Reader)
-		defer bufioReaderPool.Put(brdr)
-
 		// Fetch from object storage concurrently and update stats and posting list.
 		g.Go(func() error {
 			begin := time.Now()
 
+			brdr := bufioReaderPool.Get().(*bufio.Reader)
+			defer bufioReaderPool.Put(brdr)
+
 			partReader, err := r.block.bkt.GetRange(ctx, r.block.indexFilename(), start, length)
 			if err != nil {
 				return errors.Wrap(err, "read postings range")