The callback must be invoked, as also documented.

Otherwise, the tracked info gets leaked.

Let NMSecretAgentOld (the caller) be a bit resilient against
bugs in the client, and avoid a crash by prematurely remove
the request-info from the pending list. That does not fully
workaround the bug (it leads to a leak), but at least it does
not cause other "severe" issues.

The leak was present earlier as well.