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 58d4ec6 on adityarsuryavamshi:master
into 8785d3c on go-playground:master.

[adityarsuryavamshi]

reopened here #68