security/clamav: upgrade to 0.99.4
Release notes: http://blog.clamav.net/2018/03/clamav-0994-has-been-released.html

Submitted by: mmokhi
Security: CVE-2012-6706
Security: CVE-2017-6419
Security: CVE-2017-11423
Security: CVE-2018-1000085
Security: CVE-2018-0202
Differential Revision: https://reviews.freebsd.org/D14562