Avoid passing NULL to printf in match debug code for LDAP/SSSD.

The file name in struct userspec was not set for the LDAP and SSSD
backends.  There is no actual file in this case so set the name to
LDAP/SSSD.  Also add a guard to make sure we don't try to print
NULL in sudoers_lookup_check() if name is left unset.

plugins/sudoers/sssd.c

@@ -364,6 +364,7 @@ sss_to_sudoers(struct sudo_sss_handle *handle,
     /* We only have a single userspec */
     if ((us = calloc(1, sizeof(*us))) == NULL)
 	goto oom;
+    us->file = rcstr_dup("SSSD");
     TAILQ_INIT(&us->users);
     TAILQ_INIT(&us->privileges);
     STAILQ_INIT(&us->comments);