coturn: apply patch for CVE-2020-6061/6062

Fixes: CVE-2020-6061, CVE-2020-6062

An exploitable heap overflow vulnerability exists in the way CoTURN
4.5.1.1 web server parses POST requests. A specially crafted HTTP
POST request can lead to information leaks and other misbehavior.
An attacker needs to send an HTTPS request to trigger this vulnerability.

An exploitable denial-of-service vulnerability exists in the way
CoTURN 4.5.1.1 web server parses POST requests. A specially crafted
HTTP POST request can lead to server crash and denial of service.
An attacker needs to send an HTTP request to trigger this vulnerability.