fix inferVhdlObj behaviour when obj.min is None
fix inferVhdlObj behaviour when obj.min is None

In python 2, None < 0 returns True
However, in py3, None cannot be compared with zero. This commit ensures
identical behaviour in py2,3 when obj.min is None

jmgc/myhdl-numeric,jandecaluwe/myhdl,jck/myhdl,j-marjanovic/myhdl,jandecaluwe/myhdl,cfelton/myhdl,juhasch/myhdl,jck/myhdl,jmgc/myhdl-numeric,myhdl/myhdl,josyb/myhdl,j-marjanovic/myhdl,gw0/myhdl,cfelton/myhdl,josyb/myhdl,hgomersall/myhdl,jck/myhdl,gw0/myhdl,jandecaluwe/myhdl,hgomersall/myhdl,josyb/myhdl,hgomersall/myhdl,j-marjanovic/myhdl,cfelton/myhdl,jmgc/myhdl-numeric,gw0/myhdl,juhasch/myhdl,juhasch/myhdl,myhdl/myhdl,myhdl/myhdl