pt-tls-server: Properly fail if processing PB-TNC batches failed

Because `assess()` returns a `status_t`, `FALSE` is interpreted as
`SUCCESS`. So a failure while processing PB-TNC batches terminated
the process successfully.

Fixes: 1e92d5f114 ("Process PB-TNC batches received via PT-TLS asynchronously")

src/libpttls/pt_tls_server.c

@@ -439,7 +439,7 @@ static status_t assess(private_pt_tls_server_t *this, tls_t *tnccs)
 			case FAILED:
 			default:
 				reader->destroy(reader);
-				return FALSE;
+				return FAILED;
 			case NEED_MORE:
 				break;
 		}