Check jansson parsing function return codes for requests

[languitar]

The code for dispatching a request json to registered functions so far did not correctly check the return codes of jannson json parsing functions which allowed segmentation faults for specially crafted inputs. This PR adds the required error handling. I have found this through fuzzing the code.

I am not sure, but potentially someone could use this to craft some attacks.

Btw, why are there no issues allowed in this repo?

[languitar]

@killing @lins05 Any interst in this? As this could impact security, some kind of reaction would be great.