update postinst

DEBIAN/postinst

@@ -7,4 +7,9 @@ set -e
 db_get alice-auth/ldap-bind-password
 CONFIG=/etc/sssd/sssd.conf
 
+if [ ! -f "$CONFIG" ]; then
+    echo "Error: Configuration file $CONFIG does not exist."
+    exit 10
+fi
+
 sed -i "s/ldap_default_authtok =.*/ldap_default_authtok = $RET/" $CONFIG