login-common: Don't call client_input() directly in client_auth_failed()

login-common: Don't call client_input() directly in client_auth_failed()

Fixes a new assert-crash caused by
9aaf0554aeae4f8056eeef56cfd99bf386f4c009:

Panic: file sasl-server.c: line 357 (authenticate_callback): assertion failed: (!client->authenticating)

This was caused by a code path:
 - sasl_server_check_login()
 - sasl_server_auth_failed()
 - sasl_server_auth_cancel()
 - call_client_callback()
 - sasl_callback()
 - client_auth_failed()
 - client_input()
 - another AUTHENTICATE/LOGIN was read

The solution is then to not call client_input() directly. It would have
also worked to just remove the assert though, but this fix is cleaner.