Assert that literal vxids we parse never have the client/server bit set

This is an additional safeguard against regressions of #1762

lib/libvarnishapi/vsl_dispatch.c

@@ -696,6 +696,7 @@ vtx_parse_link(const char *str, enum VSL_transaction_e *ptype,
 		*ptype = VSL_t_unknown;
 	if (i == 1)
 		return (1);
+	assert((vxid & ~VSL_IDENTMASK) == 0);
 	*pvxid = vxid;
 	if (i == 2)
 		return (2);