mail/opendmarc: fix multiple addresses in From vulnerability

• please note that it might only be a partial fix, see https://github.com/trusteddomainproject/OpenDMARC/pull/48#issuecomment-530375590

PR: 240505
Reported by: protonmail
Approved by: ports-secteam (delphij)
Obtained from: https://github.com/trusteddomainproject/OpenDMARC/pull/48
MFH: 2019Q3
Security: https://protonmail.com/blog/bellingcat-cyberattack-phishing/