gstreamer-sdp: calling attributes_len on SDPMediaRef received from MediaIterMut segfaults

When you call attributes_len on a SDPMediaRef which you acquired via MediaIterMutthe program will segfault.

Shortest example:

use gstreamer_sdp::SDPMessage;

fn main() {
    gstreamer::init();

    let sdp = r#"
v=0
o=- 2449729779344774154 0 IN IP4 0.0.0.0
s=-
t=0
m=video 9 UDP/TLS/RTP/SAVPF 127
c=IN IP4 0.0.0.0
a=sendonly
    "#;

    let mut sdp = SDPMessage::parse_buffer(sdp.as_bytes()).unwrap();
    for media in sdp.medias_mut() {
        println!("Fetch len");
        let x = media.attributes_len();
    }
}

this occurs for me on Void Linux, with gstreamer 1.16.2 with the gstreamer-sdp crate at version 0.16.5