Bug 1429542 Make HttpChannelChild::OverrideRunnable check for nullptr before using mNewChannel. r=asuth

netwerk/protocol/http/HttpChannelChild.cpp

@@ -1500,7 +1500,9 @@ HttpChannelChild::OverrideRunnable::Run()
       mCallback = nullptr;
     }
     mChannel->CleanupRedirectingChannel(rv);
-    mNewChannel->Cancel(rv);
+    if (mNewChannel) {
+      mNewChannel->Cancel(rv);
+    }
     return NS_OK;
   }