Clear SDWebImagePrefetcher progressBlock when it has completed

If progressblock captures objects, captured objects remain until next SDWebImagePrefetcher works.
So, nil clear progressBlock as well as completionBlock.

SDWebImage/SDWebImagePrefetcher.m

@@ -94,6 +94,7 @@
                 self.completionBlock(self.finishedCount, self.skippedCount);
                 self.completionBlock = nil;
             }
+            self.progressBlock = nil;
         }
     }];
 }