Fix a race condition during progressive animation load in SDAnimatedImageView.
Fix a race condition during progressive animation load in SDAnimatedImageView.

When the coder was updated, currentData may not be the same instance as previousdData. We should check the that current data is appended by previous data

awkward/SDWebImage,dreampiggy/SDWebImage,tualatrix/SDWebImage,awkward/SDWebImage,awkward/SDWebImage,dreampiggy/SDWebImage,dreampiggy/SDWebImage,rs/SDWebImage,zhang-yawei/SDWebImage