[Autofill Auth] Updating WebAuthn offer dialog only when shown.

The registration dialog is only shown when offering opt-in and during the fetch
challenge call. Any other scenarios, it is not showing. This fix updates the
dialog only when it is showing, to avoid any nullptr issues.

Bug: 949269
Change-Id: I26b850ac4e7e51bf32a62a2a967a803b3ffe9479
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1885101Reviewed-by: Jared Saul <jsaul@google.com>
Commit-Queue: Manas Verma <manasverma@google.com>
Cr-Commit-Position: refs/heads/master@{#710183}

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

@@ -340,7 +340,7 @@ void CreditCardFIDOAuthenticator::OnDidGetOptChangeResult(
          current_flow_ == FOLLOWUP_AFTER_CVC_AUTH_FLOW);
   // End the flow if the server responded with an error.
   if (result != AutofillClient::PaymentsRpcResult::SUCCESS) {
-    if (current_flow_ != OPT_OUT_FLOW)
+    if (current_flow_ == OPT_IN_FETCH_CHALLENGE_FLOW)
       autofill_client_->UpdateWebauthnOfferDialogWithError();
     current_flow_ = NONE_FLOW;
     return;