Problem:
  "parser.head" is not NULL even if it's free'ed by "virJSONValueFree",
returning "parser.head" when "virJSONValueFromString" fails will cause
unexpected errors (libvirtd will crash sometimes), e.g.
  In function "qemuMonitorJSONArbitraryCommand":

        if (!(cmd = virJSONValueFromString(cmd_str)))
            goto cleanup;

        if (qemuMonitorJSONCommand(mon, cmd, &reply) < 0)
            goto cleanup;

        ......

     cleanup:
        virJSONValueFree(cmd);

  It will continues to send command to monitor even if "virJSONValueFromString"
is failed, and more worse, it trys to free "cmd" again.

  Crash example:
{"error":{"class":"QMPBadInputObject","desc":"Expected 'execute' in QMP input","data":{"expected":"execute"}}}
{"error":{"class":"QMPBadInputObject","desc":"Expected 'execute' in QMP input","data":{"expected":"execute"}}}
error: server closed connection:
error: unable to connect to '/var/run/libvirt/libvirt-sock', libvirtd may need to be started: Connection refused
error: failed to connect to the hypervisor

  This fix is to:
    1) return NULL for failure of "virJSONValueFromString",
    2) and it seems "virJSONValueFree" uses incorrect loop index for type
       of "VIR_JSON_TYPE_OBJECT", fix it together.

* src/util/json.c