Update multi_byte.rs

[dongbin86]

Under some extreme circumstances, the pre-allocated length might not be sufficient.

#[test]
    fn test2() {
        let input = "?gb2312?B?oQ==?=";
        match MessageStream::new(input.as_bytes()).decode_rfc2047() {
            Some(result) => {
                println!("Decoded '{}'", result);
                //assert_eq!(result, expected_result);
            }
            _ => panic!("Failed to decode '{}'", input),
        
        }
    }

[mdecimus]

Vectors in rust can grow in size dynamically, this is not needed.

[dongbin86]

@mdecimus sorry, but you can try it out, if not modified like me, it will directly throw an error saying "String full while decoding."

[mdecimus]

Thank you @dongbin86 - the encoder_rs crate is not allowing the vector to grow automatically, so your fix is correct.