Update SDWebImageDownloaderOperation.m

Fixing race condition that causes cancellation of one download operation to stop a run loop that is now used for another download operation. Race is introduced through -performSelector:onThread:withObject:waitUntilDone:

SDWebImage/SDWebImageDownloaderOperation.m

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