Validate hangs with deeply nested arrays

[WillAbides]

This bit of invalid json causes Validate to hang:

https://github.com/nst/JSONTestSuite/blob/master/test_parsing/n_structure_100000_opening_arrays.json

[kevburnsjr]

I saw the same behavior while implementing #68.
Recommend enforcing 500 depth (as global var) for both parse and validate.