Port Max's crash fix to my 3.6 fork

@@ -84,7 +84,9 @@
         }
 
         if (self.prefetchURLs.count > self.requestedCount) {
+            dispatch_async(dispatch_get_main_queue(), ^{
                 [self startPrefetchingAtIndex:self.requestedCount];
+            });
         }
         else if (self.finishedCount + self.skippedCount == self.requestedCount) {
             [self reportStatus];