dnp3: use base64 macro in gen script

As is done already in C
cf commit ea0936199d

(cherry picked from commit 5ec9688f03)

scripts/dnp3-gen/dnp3-gen.py

@@ -171,7 +171,7 @@ void OutputJsonDNP3SetItem(JsonBuilder *js, DNP3Object *object,
 {% elif field.type in ["flt32", "flt64"] %}
             jb_set_float(js, "{{field.name}}", data->{{field.name}});
 {% elif field.type == "bytearray" %}
-            unsigned long {{field.name}}_b64_len = data->{{field.len_field}} * 2;
+            unsigned long {{field.name}}_b64_len = BASE64_BUFFER_SIZE(data->{{field.len_field}});
             uint8_t {{field.name}}_b64[{{field.name}}_b64_len];
             Base64Encode(data->{{field.name}}, data->{{field.len_field}},
                 {{field.name}}_b64, &{{field.name}}_b64_len);