Without the fix we can have a race, example:
```
Write at 0x00c000432039 by goroutine 187:
github.com/rclone/rclone/fs/accounting.(*StatsInfo).Error()
fs/accounting/stats.go:495 +0x3f1
github.com/rclone/rclone/fs/accounting.(*StatsInfo).Error-fm()
fs/accounting/stats.go:477 +0x55
github.com/rclone/rclone/fs/walk.listRwalk.func1()
fs/walk/walk.go:162 +0xd2
github.com/rclone/rclone/fs/walk.walk.func2()
fs/walk/walk.go:402 +0x30f
Previous read at 0x00c000432039 by goroutine 184:
github.com/rclone/rclone/fs/accounting.(*statsGroups).sum()
fs/accounting/stats_groups.go:351 +0xcae
github.com/rclone/rclone/fs/accounting.rcTransferredStats()
fs/accounting/stats_groups.go:132 +0x1f4
```
Fixes#3844