[FastRoute] Fix layer restriction when design has less than 3 layers
[FastRoute] Fix layer restriction when design has less than 3 layers

QuantamHD/OpenROAD,The-OpenROAD-Project/OpenROAD,QuantamHD/OpenROAD,The-OpenROAD-Project/OpenROAD,The-OpenROAD-Project/OpenROAD,QuantamHD/OpenROAD,QuantamHD/OpenROAD,QuantamHD/OpenROAD,The-OpenROAD-Project/OpenROAD,The-OpenROAD-Project/OpenROAD