rs/SDWebImage — pull request #647 by musaicapp (2014-03-04 04:25:33+00:00)
Title: Prefetcher: check progressBlock is not nil before executing.

Currently, the following prefetch call will crash:
```
[[SDWebImagePrefetcher sharedImagePrefetcher] prefetchURLs: urls];
```
because it on-calls [prefetchURLs:progress:completed:] with nil blocks, but does not check this before attempting to execute them.