Merge pull request #1017 from warld-limited/prefetcher_patch

[Memory Issue] Clear SDWebImagePrefetcher progressBlock when it has completed

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