Type definition with internal underlying type leads to panic

[greenpart]

Describe the bug
swag panics trying to generate documentation for type defined with internal underlying type (e.g. sql.NullInt64).

panic: runtime error: invalid memory address or nil pointer dereference
…
exit status 2

To Reproduce
Steps to reproduce the behavior:

1. create file in an empty directory and fill it with:

package main

import (
	"database/sql"
)

// @Success 200 {object}  main.Object "Response data"
func main() {
	_ = Object{}
}

type Object struct {
	ID NullableID
}

type NullableID sql.NullInt64

2. run swag init --parseDependency in this directory
3. See error panic: runtime error: invalid memory address or nil pointer dereference

Expected behavior
Swagger documentation successfully generated.

Your swag version
1.6.2

Your go version
1.12.3

Desktop (please complete the following information):

• OS X 10.12.6

[easonlin404]

it's should be use swagger type tag for non supporting type.

[greenpart]

Tags are not applicable with type definitions like type NullableID sql.NullInt64.
We have some types like that and want to continue with them rather than convert to

type NullableID struct {
    sql.NullInt64 `swaggertype:"…"`
}