Fix that the downloader operation may not call the completion block in race condition, which the operation we get is finished but not been removed from the operation array.
Fix that the downloader operation may not call the completion block in race condition, which the operation we get is finished but not been removed from the operation array.

awkward/SDWebImage,tualatrix/SDWebImage,awkward/SDWebImage,zhang-yawei/SDWebImage,smule/SDWebImage,Insfgg99x/SDWebImage,rs/SDWebImage,awkward/SDWebImage,dreampiggy/SDWebImage,dreampiggy/SDWebImage,dreampiggy/SDWebImage,iheartradio/SDWebImage