[Payments Autofill] Avoiding nullptr crashes in Payments Client

Adding a nullptr check to OnSimpleLoaderCompleteInternal() to avoid crashes
that have been appearing since M69.

Change-Id: Iaa1ddc1cefb741917029bbfb581995c7836d373a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1949549Reviewed-by: Jared Saul <jsaul@google.com>
Commit-Queue: Manas Verma <manasverma@google.com>
Cr-Commit-Position: refs/heads/master@{#721687}

components/autofill/core/browser/payments/payments_client.cc

@@ -1241,6 +1241,9 @@ void PaymentsClient::OnSimpleLoaderCompleteInternal(int response_code,
 
   AutofillClient::PaymentsRpcResult result = AutofillClient::SUCCESS;
 
+  if (!request_)
+    return;
+
   switch (response_code) {
     // Valid response.
     case net::HTTP_OK: {