Fix #880 Max recursion depth for cJSON_Duplicate to prevent stack exhaustion

[vwvw]

Dear maintainers,

Here is a suggestion of a fix to prevent the stack exhaustion in case of circular reference.

I duplicated the cJSON_Duplicate so that it could take a depth argument. This one is compared to CJSON_CIRCULAR_LIMIT which is currently set to 10'000 but likely need adaption based on your knowledge of what is use in practice.

I also added a test under cjson_should_not_follow_too_deep_circular_references.

If you don't like the extra function cJSON_Duplicate_rec, this fix could also be implemented through an additional field in the cJSON struct (likely break the ABI?) or through a global variable (making cJSON more thread unsafe).

What is not yet fixed

In addition to cJSON_Duplicate, the same issue happens in cJSON_Delete. I have a similar fix for it too but I'm unsure on what to do once we reach CJSON_CIRCULAR_LIMIT. Aborting or exiting looks like the correct solution as it is likely that memory corruption occurred through a double free.

What would you suggest to do in this case?

[vwvw]

This would fix #880

[PeterAlfredLee]

Looks good to me for now.

This PR changes the original behavior of cJSON_Duplicate. For nested objects with a depth of over 10,000(which is deep enough IMO), cJSON_Duplicate would stop.
If this makes any inconvenience to anyone, feel free to tell me.