screen: crash when processing combining chars (CVE-2021-26937)

encoding.c in GNU Screen through 4.8.0 allows remote attackers to cause a denial of service (invalid write access and application crash) or possibly have unspecified other impact via a crafted UTF-8 character sequence.

References:

• https://www.openwall.com/lists/oss-security/2021/02/09/3
• https://nvd.nist.gov/vuln/detail/CVE-2021-26937

Affected branches:

• master
• 3.13-stable
• 3.12-stable
• 3.11-stable
• 3.10-stable