validation crash on missing nested required field

[itaischwartz]

Hey @goccy,

I've run the following test:

package main 

import (
	"strings"

	"github.com/goccy/go-yaml"
	"gopkg.in/go-playground/validator.v9"
)

type Person struct {
	Name string `yaml:"name" validate:"required"`
	Age  int    `yaml:"age" validate:"gte=0,lt=120"`
	Addr *PersonAddress `yaml:"addr" validate:"required,dive,required"`
}
type PersonAddress struct {
	Number string `yaml:"number" validate:"required"`
	State string `yaml:"state" validate:"required"`
}


func main() {
	yml := `
name: itai
age: 10
addr:
  number: seven
` // missing State
	validate := validator.New()
	dec := yaml.NewDecoder(
		strings.NewReader(yml),
		yaml.Validator(validate),
	)
	var v Person
	err := dec.Decode(&v)
	if err == nil {
		panic("expected error")
	}
}

got the following crash log:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x10effb2]

goroutine 1 [running]:
github.com/goccy/go-yaml.(*Decoder).decodeStruct(0xc0001fe000, 0x11b2480, 0xc0000a9fc0, 0x199, 0x1210c20, 0xc000105a80, 0x28, 0x11aa560)
        /Users/itai/work/go-yaml/decode.go:855 +0x1b12
github.com/goccy/go-yaml.(*Decoder).decodeValue(0xc0001fe000, 0x11b2480, 0xc0000a9fc0, 0x199, 0x1210c20, 0xc000105a80, 0x0, 0x2a)
        /Users/itai/work/go-yaml/decode.go:473 +0x1b0a
github.com/goccy/go-yaml.(*Decoder).decode(0xc0001fe000, 0x1189e00, 0xc0000a9fc0, 0x16, 0x1189e00, 0x1337a20)
        /Users/itai/work/go-yaml/decode.go:1243 +0xd2
github.com/goccy/go-yaml.(*Decoder).Decode(0xc0001fe000, 0x1189e00, 0xc0000a9fc0, 0x1006cbf, 0xc00008e058)
        /Users/itai/work/go-yaml/decode.go:1272 +0x272
main.main()
        /Users/itati/work/go-yaml/cmd/validate/cmd.go:34 +0x1f6
exit status 2

shell returned 1

It seems that structFieldMap doesn't contain "State" fieldName, so we crash on null pointer deref (structField.RenderName)

					structField := structFieldMap[fieldName]
					node, exists := keyToNodeMap[structField.RenderName]