Fix crash:23193f541b449918

This CL prevents a crash caused by dereferencing a null `WeakPtr` in
`CreditCardRiskBasedAuthenticator::OnUnmaskCancelled`.

The crash occurs when `OnUnmaskCancelled` is called after the
`Requester` object (e.g., `CreditCardAccessManager`) has been
deallocated, which can happen during the dismissal flow of UI elements
like the Autofill progress dialog. The `requester_` member is a
`base::WeakPtr`, and accessing it after the target is destroyed leads to
a use-after-free.

The fix adds a check to ensure the `requester_` `WeakPtr` is still valid
before dereferencing it to call
`OnRiskBasedAuthenticationResponseReceived`. This prevents the crash by
avoiding the call on a dangling pointer.

This issue was observed in crash report crash/23193f541b449918.

Bug: 446729816

Change-Id: I271a780a5c7b0653300e60c08bd7d1525248fd0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6967944
Reviewed-by: Siyu An <siyua@chromium.org>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Yiwen Qian <yiwenqian@google.com>
Cr-Commit-Position: refs/heads/main@{#1519572}