Properties are checked twice for each undefined boolean constants

[merkste]

If a model contains an undefined boolean constant that is assigned true or false at the command line, all properties are checked twice. Consider the following example:

// A simple model that depends on an undefined boolean constant
mdp

const bool flag;

module Boolean
  b : bool init flag;
endmodule

and the command line

prism example.prism -pf b -const flag=false

[davexparker]

Fixed by #233.