Improvement for #1608 and #1623 should solve the issue for most of the cases. Ap…

…parently there is a race condition on NSURLCache and we avoid making those checks unless necesarry (basically we will query the NSURLCache only when `SDWebImageRefreshCached` is used and the image cannot be cached by the system when it's too big or behind authentication)

@@ -404,12 +404,14 @@ didReceiveResponse:(NSURLResponse *)response
     } else {
         SDWebImageDownloaderCompletedBlock completionBlock = self.completedBlock;
         
-        if (![[NSURLCache sharedURLCache] cachedResponseForRequest:self.request]) {
-            responseFromCached = NO;
-        }
-        
         if (completionBlock) {
-            if (self.options & SDWebImageDownloaderIgnoreCachedResponse && responseFromCached) {
+            /**
+             *  See #1608 and #1623 - apparently, there is a race condition on `NSURLCache` that causes a crash
+             *  Limited the calls to `cachedResponseForRequest:` only for cases where we should ignore the cached response
+             *    and images for which responseFromCached is YES (only the ones that cannot be cached).
+             *  Note: responseFromCached is set to NO inside `willCacheResponse:`. This method doesn't get called for large images or images behind authentication 
+             */
+            if (self.options & SDWebImageDownloaderIgnoreCachedResponse && responseFromCached && [[NSURLCache sharedURLCache] cachedResponseForRequest:self.request]) {
                 completionBlock(nil, nil, nil, YES);
             } else if (self.imageData) {
                 UIImage *image = [UIImage sd_imageWithData:self.imageData];