Prevent assert(0) in rlm_rest

If the RADIUS server is compiled without JSON support, it is still possible that the other end of the rlm_rest module returns JSON. This triggered an "assert(0)" and crashed the server. This fixes this by printing an error message to indicate that the reply of the REST server couldn't be parsed, and failing the module gracefully.
Merge pull request #808 from qnet-herwin/rlm_rest_json_from_server

Prevent assert(0) in rlm_rest