mail/fetchmail: security update to 6.4.20

Security: cbfd1874-efea-11eb-8fe9-036bd763ff35
Security: CVE-2021-36386
Approved by: Corey Halpin (maintainer)
PR: 257480
MFH: 2021Q3

(cherry picked from commit dd81af38c7e2c6ab123601a2e524c99a20eb704d)