core: don't call nm_auth_chain_destroy() from the callback

NMAuthChain is not ref-counted.

You may call nm_auth_chain_destroy() once before the callback
gets invoked. This destroys the auth-chain instance right away.

You may call nm_auth_chain_destroy() once from inside the callback.
This basically has no effect but is allowed for convenince.
All this does is remembering that destroy was called and asserts that
destroy gets called at most once.

After the callback returns, the auth-chain will always be destroyed.
That means, generally there is no need to call nm_auth_chain_destroy()
from inside the callback.

Remove that code, and refactor some code to return early (where it makes
sense).

src/nm-auth-utils.c

@@ -355,11 +355,14 @@ nm_auth_chain_new_subject (NMAuthSubject *subject,
  * Destroys the auth-chain. By destroying the auth-chain, you also cancel
  * the receipt of the done-callback. IOW, the callback will not be invoked.
  *
- * The only exception is, if may call nm_auth_chain_destroy() from inside
- * the callback. In this case, @self stays alive until the callback returns.
+ * The only exception is, you may call nm_auth_chain_destroy() from inside
+ * the callback. In this case the call has no effect and @self stays alive
+ * until the callback returns.
  *
  * Note that you might only destroy an auth-chain exactly once, and never
- * after the callback was handled.
+ * after the callback was handled. After the callback returns, the auth chain
+ * always gets automatically destroyed. So you only need to explicitly destroy
+ * it, if you want to abort it before the callback complets.
  */
 void
 nm_auth_chain_destroy (NMAuthChain *self)