[Payments] Prevent nullptr dereference

Bug: 853950
Change-Id: I39129f9adb62ca3a0298fc20cb60751959e4ebdd
Reviewed-on: https://chromium-review.googlesource.com/1110343
Commit-Queue: Ganggui Tang <gogerald@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569364}

components/payments/content/utility/payment_manifest_parser.cc

@@ -449,7 +449,7 @@ void PaymentManifestParser::OnWebAppParseInstallationInfo(
     std::unique_ptr<base::Value> value) {
   // TODO(crbug.com/782270): Move this function into a static function for unit
   // test.
-  if (value->FindKey({kServiceWorker}) == nullptr) {
+  if (!value || value->FindKey({kServiceWorker}) == nullptr) {
     return std::move(callback).Run(nullptr, nullptr);
   }