Fix panic when the decoding pointer type with custom decoding function

[adityarsuryavamshi]

Fixes Or Enhances

When a pointer type has a custom decoder and the decoded value is nil, we get a panic.

Example

func main() {
	type CustomTime *time.Time
	type TestError struct {
		CT CustomTime
	}
	var test TestError
	decoder = form.NewDecoder()
	decoder.RegisterCustomTypeFunc(func(s []string) (interface{}, error) {
		if s[0] == "" {
			return nil, nil
		}
		parsed, err := time.Parse(time.RFC3339, s[0])
		if err != nil {
			return nil, err
		}
		return CustomTime(&parsed), nil
	}, CustomTime(nil))

	values := url.Values{
		"CT": []string{""},
	}
	decoder.Decode(&test, values)
}

panic: reflect: call of reflect.Value.Set on zero Value

This PR specifically handles the case for the case when the type is a pointer.

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

@go-playground/admins

[coveralls]

coverage: 99.815% (+0.001%) from 99.814%
when pulling 3618b1a on adityarsuryavamshi:fix-pointer-decoder-panic
into 8785d3c on go-playground:master.

[deankarn]

Awesome PR TY!

[adityarsuryavamshi]

@deankarn Thank you so much for the quick response. This PR was raised to fix a specific issue I had with the pointer types. There might be other scenarios (specifically with type declarations of slices and interfaces) where the same issue might occur, I have neither ran across them nor tested for them.

[acynothia]

@deankarn It's a breaking change. see case below.

type User struct {
   Name string
}

decoder.RegisterCustomTypeFunc(
  func(vals []string) (any, error) {
    return &User{Name: vals[0]}
  }, &User{},
)

[deankarn]

@acynothia can you provide an example I can run with input 🙏

in the meantime @adityarsuryavamshi I will likely have to rollback the change.

[deankarn]

Changes have been reverted/retracted in https://github.com/go-playground/form/releases/tag/v4.2.3 until I can get an example from @acynothia that I can test this change with to try and find a way to have both work successfully.

[acynothia]

please have a glance at this case, it working well in previous version. https://go.dev/play/p/h6N0oYCZuNP