Merge pull request #698 from salling/master

Fixing race in SDWebImageDownloaderOperation leading to erroneous timeout.

@@ -129,6 +129,7 @@
 }
 
 - (void)cancelInternalAndStop {
+    if (self.isFinished) return;
     [self cancelInternal];
     CFRunLoopStop(CFRunLoopGetCurrent());
 }