Json parser can truncate large numbers

zbx_json_decodevalue function truncates numeric value if the passed buffer size is less than the value length.

It does not seem to affect normal Zabbix work (external data is parsed into string before converting to numeric tupes, for internal communications the buffers used to parse numeric values are large enough for data generated by Zabbix itself).

There might be issues with third party applications using Zabbix internal protocols. And of course it's a land mine we could step on in future, so it has to be fixed.