Bug 598790 - nullcheck canceled channel in prefetchnode::onstoprequest r=bz

uriloader/prefetch/nsPrefetchService.cpp

@@ -217,7 +217,7 @@ nsPrefetchNode::OnStopRequest(nsIRequest *aRequest,
 {
     LOG(("done prefetching [status=%x]\n", aStatus));
 
-    if (mBytesRead == 0 && aStatus == NS_OK) {
+    if (mBytesRead == 0 && aStatus == NS_OK && mChannel) {
         // we didn't need to read (because LOAD_ONLY_IF_MODIFIED was
         // specified), but the object should report loadedSize as if it
         // did.