Function alignment_32 

pub fn alignment_32(size: usize) -> usize

compute padding size.

RFC5766 stipulates that the attribute content is a multiple of 4.

Test

use turn_server::codec::message::alignment_32;

assert_eq!(alignment_32(4), 0);
assert_eq!(alignment_32(0), 0);
assert_eq!(alignment_32(5), 3);