Fix blank images during fast swiping.

When I tested some local images on iPhone 5s, it was OK. But when I
tested them on iPhone 4, many time-outs occured and I saw many blank
images.

````
SDWebImage failed to download image: Error Domain=NSURLErrorDomain
Code=-1001 "The operation couldn’t be completed. (NSURLErrorDomain error
-1001.)" UserInfo=0x175a3530
{NSErrorFailingURLKey=file:///var/mobile/Applications/CDAFFE04-7BE4-4118-AA79-263718CFCAF0/Documents/66-jpgs/66_21.jpg}
````