[rtler] fixed bug in port_walk, slice issue when no ValueNodes
[rtler] fixed bug in port_walk, slice issue when no ValueNodes

cornell-brg/pymtl,tj93/pymtl,12yujim/pymtl,jck/pymtl,cfelton/pymtl,Glyfina-Fernando/pymtl,Abhinav117/pymtl,cornell-brg/pymtl,jck/pymtl,12yujim/pymtl,Glyfina-Fernando/pymtl,12yujim/pymtl,Abhinav117/pymtl,jck/pymtl,Glyfina-Fernando/pymtl,tj93/pymtl,Abhinav117/pymtl,cfelton/pymtl,cornell-brg/pymtl,cfelton/pymtl,jjffryan/pymtl,tj93/pymtl,Abhinav117/pymtl,jjffryan/pymtl,jjffryan/pymtl