fix address nil values

lib/risc/text_writer.rb

@@ -138,7 +138,8 @@ module Risc
     def write_return_address( addr )
       write_ref_for( addr.get_type )
       write_ref_for( addr.next_integer )
-      write_ref_for( addr.value + @linker.platform.loaded_at )
+      val = addr.value ? addr.value + @linker.platform.loaded_at : Parfait.object_space.nil_object
+      write_ref_for( val )
       write_ref_for( 0 )
       log.debug "Integer witten stream 0x#{@stream.length.to_s(16)}"
     end

test/risc/interpreter/calling/test_minus.rb

@@ -45,7 +45,7 @@ module Risc
       ret = main_ticks(64)
       assert_equal FunctionReturn ,  ret.class
       assert_equal :r1 ,  ret.register.symbol
-      assert_equal 41992 ,  @interpreter.get_register(ret.register)
+      assert_equal 50632 ,  @interpreter.get_register(ret.register)
     end
   end
 end