Bug 1917899 - Check unsupported import attributes before resolving specifier r=jonco

We need to check if we have an unsupported attribute before resolving the
specifier because, according to the spec, the unknown attribute should be
reported before the invalid specifier.

Differential Revision: https://phabricator.services.mozilla.com/D221998

js/loader/ModuleLoaderBase.cpp

@@ -839,7 +839,15 @@ nsresult ModuleLoaderBase::ResolveRequestedModules(
   for (uint32_t i = 0; i < length; i++) {
     JS::Rooted<JSString*> str(
         cx, JS::GetRequestedModuleSpecifier(cx, moduleRecord, i));
-    MOZ_ASSERT(str);
+    if (!str) {
+      JS::Rooted<JS::Value> pendingException(cx);
+      if (!JS_GetPendingException(cx, &pendingException)) {
+        return NS_ERROR_FAILURE;
+      }
+      ms->SetParseError(pendingException);
+      JS_ClearPendingException(cx);
+      return NS_ERROR_FAILURE;
+    }
 
     nsAutoJSString specifier;
     if (!specifier.init(cx, str)) {