crashfix: SDImageIOAnimatedCoder crash

Author: jianglei

SDWebImage/Private/SDImageFramePool.m

@@ -109,13 +109,16 @@ - (void)prefetchFrameAtIndex:(NSUInteger)index {
 
     if (self.fetchQueue.operationCount == 0) {
         // Prefetch next frame in background queue
-        id<SDAnimatedImageProvider> animatedProvider = self.provider;
         @weakify(self);
         NSOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
             @strongify(self);
             if (!self) {
                 return;
             }
+            id<SDAnimatedImageProvider> animatedProvider = self.provider;
+            if (!animatedProvider) {
+                return;
+            }
             UIImage *frame = [animatedProvider animatedImageFrameAtIndex:index];
 
             [self setFrame:frame atIndex:index];