Add rudimentary buffer overrun protection

[Caleb-Daniels-SFC]

I love your library and its been incredibly useful for getting started on using json. I have had to add some rudimentary buffer overrun protection to ensure this works on embedded systems. Its not perfect by any means and I would not add this in its current state but it should lay some ground work for some basic protection from buffer overruns. I was unsure of a better way to do it but it works for the most part. I tried following your coding standards and style to the best of my ability