Bug 1729909 - Fix crash in Document::LocalizationLinkAdded; r=platform-i18n-reviewers,zbraniecki

If for some reason we can't create the promise for mDocumentL10n, we should
return early rather than crashing.

Differential Revision: https://phabricator.services.mozilla.com/D125234

dom/base/Document.cpp

@@ -4171,7 +4171,9 @@ void Document::LocalizationLinkAdded(Element* aLinkElement) {
 
     bool isSync = elem->HasAttr(nsGkAtoms::datal10nsync);
     mDocumentL10n = DocumentL10n::Create(this, isSync);
-    MOZ_ASSERT(mDocumentL10n);
+    if (NS_WARN_IF(!mDocumentL10n)) {
+      return;
+    }
   }
   mDocumentL10n->AddResourceId(NS_ConvertUTF16toUTF8(href));