add a new option to json_file constructor

[jacobwilliams]

If nullify_pointer is False, the pointer is not nullified
Fixes #564

[LaplaceSoda]

Test results show that after adding nullify_pointer=.false., the original json_file object seems still broken.

[jacobwilliams]

Yeah, this doesn't really "fix" the issue you mention in #564, but it just gives another option for something somebody might want to do in some cases. Specifically, add a json pointer to a json_file, but keeping it associated, and then subsequently nullifying the one in the json_file.