use fake memory

fix integer offset bug
(which only didn’t cause errors as fixnums are still an order too big
and the famous +1 error hit the empty space)

lib/risc/fake_memory.rb

@@ -34,7 +34,7 @@ module Risc
 
     def range_check(index)
       raise "index too low #{index} < #{min}" if index < min
-      raise "index too big #{index} < #{min}" if index >= size
+      raise "index too big #{index} >= #{size}" if index >= size
     end
   end
 end