Bug 1602051: Failing to pass an observer to imgITools.decodeImageFromChannelAsync crashes. r=snorp

Differential Revision: https://phabricator.services.mozilla.com/D56222

--HG--
extra : moz-landing-system : lando

image/imgTools.cpp

@@ -86,7 +86,7 @@ class ImageDecoderListener final : public nsIStreamListener,
       : mURI(aURI),
         mImage(nullptr),
         mCallback(aCallback),
-        mObserver(new NotificationObserverWrapper(aObserver)) {
+        mObserver(aObserver ? new NotificationObserverWrapper(aObserver) : nullptr) {
     MOZ_ASSERT(NS_IsMainThread());
   }