Add missing return when user is not found in sudo_sss_result_get().

Previously we fell through to the default case which just logged a
debug message and returned so this just avoids the extra (generic)
debug message.

plugins/sudoers/sssd.c

@@ -791,6 +791,7 @@ sudo_sss_result_get(struct sudo_nss *nss, struct passwd *pw, uint32_t *state)
 	    break;
 	case ENOENT:
 	    sudo_debug_printf(SUDO_DEBUG_INFO, "The user was not found in SSSD.");
+	    debug_return_ptr(NULL);
 	default:
 	    sudo_debug_printf(SUDO_DEBUG_INFO, "sss_error=%u\n", sss_error);
 	    debug_return_ptr(NULL);