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Fix panic when the decoding pointer type with custom decoding function (#68)

## Fixes Or Enhances

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

Example 

```go
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:**
- [x] Tests exist or have been written that cover this particular
change.

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