Bug 1207312: P1. Prevent crash when more than one promise is rejected. r=bholley

xpcom/threads/MozPromise.h

@@ -237,6 +237,11 @@ private:
 
     void Reject(const RejectValueType& aRejectValue)
     {
+      if (!mPromise) {
+        // Already rejected.
+        return;
+      }
+
       mPromise->Reject(aRejectValue, __func__);
       mPromise = nullptr;
       mResolveValues.Clear();