build error `json.encode()` optional field.

V version: 0.3.2 ae816b1
OS: ArchLinux

What did you do?

import json

struct Foo{
	name string
	num  ?int
}

fn main(){
	f := Foo{}
	dump(json.encode(f))
}

What did you expect to see?
No error.
Ideally, the following results.
{"name":""} or {"name":"","num":null}

What did you see instead?

==================
/tmp/v_1000/bug.330116750381110337.tmp.c:2155: error: '_result__option_int' undeclared
...
==================

import json

struct Foo{
	name string
	num  ?int = none
}

fn main(){
	f := Foo{}
	dump(json.encode(f))
}

> v run bug.v
V panic: array.get: index out of range (i == -1, a.len == 0)
v hash: ae816b1
/tmp/v_1000/v2.440370384817743901.tmp.c:19849: at _v_panic: Backtrace
/tmp/v_1000/v2.440370384817743901.tmp.c:19352: by array_get
/tmp/v_1000/v2.440370384817743901.tmp.c:20749: by v__gen__c__Gen_nth_stmt_pos
/tmp/v_1000/v2.440370384817743901.tmp.c:20759: by v__gen__c__Gen_go_before_stmt
/tmp/v_1000/v2.440370384817743901.tmp.c:6696: by v__gen__c__Gen_expr_with_tmp_var
/tmp/v_1000/v2.440370384817743901.tmp.c:20527: by v__gen__c__Gen_zero_struct_field
/tmp/v_1000/v2.440370384817743901.tmp.c:20470: by v__gen__c__Gen_struct_init
/tmp/v_1000/v2.440370384817743901.tmp.c:8244: by v__gen__c__Gen_expr
/tmp/v_1000/v2.440370384817743901.tmp.c:5852: by v__gen__c__Gen_expr_string
/tmp/v_1000/v2.440370384817743901.tmp.c:17673: by v__gen__c__Gen_gen_jsons
/tmp/v_1000/v2.440370384817743901.tmp.c:5136: by v__gen__c__gen
/tmp/v_1000/v2.440370384817743901.tmp.c:35050: by v__builder__cbuilder__gen_c
/tmp/v_1000/v2.440370384817743901.tmp.c:35068: by v__builder__cbuilder__build_c
/tmp/v_1000/v2.440370384817743901.tmp.c:35029: by v__builder__cbuilder__compile_c
/tmp/v_1000/v2.440370384817743901.tmp.c:34940: by v__builder__Builder_rebuild
/tmp/v_1000/v2.440370384817743901.tmp.c:34138: by v__builder__compile
/tmp/v_1000/v2.440370384817743901.tmp.c:35377: by main__rebuild
/tmp/v_1000/v2.440370384817743901.tmp.c:35264: by main__main
/tmp/v_1000/v2.440370384817743901.tmp.c:36323: by main

#16447

[enghitalo]

fixed in json2