fix the gapping of instructions

so last binary stays empty and new starts at 8

test/risc/position/test_instruction_listener.rb

@@ -23,6 +23,7 @@ module Risc
       assert_raises {InstructionListener.init(nil , @binary)}
     end
     def test_listener_method
+      @position.set(8)
       listener = InstructionListener.new( @instruction , @binary )
       listener.position_changed(@position)
     end

test/risc/position/test_position.rb

@@ -10,6 +10,9 @@ module Risc
     def test_new
       assert @pos
     end
+    def test_invalid
+      assert !@pos.valid?
+    end
     def test_next_slot
       mov = Arm::ArmMachine.mov(:r1 , :r1)
       position = Position.new(mov , 0)