Bug 1594147 - prevent possible null dereference in MaybeDispatchPrematureAbortEvents, r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D52409

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

dom/xhr/XMLHttpRequestWorker.cpp

@@ -1529,6 +1529,12 @@ void XMLHttpRequestWorker::MaybeDispatchPrematureAbortEvents(ErrorResult& aRv) {
       return;
     }
 
+    // Similarly to null check in ::Open, mProxy may have been cleared here.
+    if (!mProxy) {
+      aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
+      return;
+    }
+
     mProxy->mSeenUploadLoadStart = false;
   }