add a function json_valid which returns true if the argument is valid json

[db48x]

I found that my queries were incomplete unless I could differentiate between an invalid object and an object that didn't have the property I was looking for.

The code could probably be simplified; I welcome any feedback you may have.

[kazuho]

Sorry for not responding. It would be great if you could take the approach described below instead of copying the code.

• convert and rename json_get to an internal function that returns both the result and error message
• json_get and json_is_valid (personally I prefer functions returning booleans be prefixed with is) should be a wrapper function calling the internal function