Merge pull request #929 from kirualex/patch-2

Handle empty urls NSArray

SDWebImage/SDWebImagePrefetcher.m

@@ -120,11 +120,17 @@
     self.completionBlock = completionBlock;
     self.progressBlock = progressBlock;
 
+    if(urls.count == 0){ 
+        if(completionBlock){
+            completionBlock(0,0);
+        }
+    }else{
         // Starts prefetching from the very first image on the list with the max allowed concurrency
         NSUInteger listCount = self.prefetchURLs.count;
         for (NSUInteger i = 0; i < self.maxConcurrentDownloads && self.requestedCount < listCount; i++) {
             [self startPrefetchingAtIndex:i];
         }
+    }
 }
 
 - (void)cancelPrefetching {