pam_fprintd: Consistently return PAM_AUTHINFO_UNAVAIL when device has no prints

Loading saved prints may lead to an error if they were stored long time
ago and so they're using a wrong format.

In such case we list the prints as available even though they are really
not, so the PAM module won't return PAM_AUTHINFO_UNAVAIL as in the
no-prints case but PAM_USER_UNKNOWN.

This will lead some auth systems (such as gdm) to keep retrying using
PAM fprintd module, even if it's not really available.

pam/pam_fprintd.c

@@ -351,7 +351,7 @@ verify_started_cb (sd_bus_message *m,
       if (sd_bus_error_has_name (error, "net.reactivated.Fprint.Error.NoEnrolledPrints"))
         {
           pam_syslog (data->pamh, LOG_DEBUG, "No prints enrolled");
-          data->verify_ret = PAM_USER_UNKNOWN;
+          data->verify_ret = PAM_AUTHINFO_UNAVAIL;
         }
       else
         {