dns/dnsmasq: Fix TFTP regression on recent FreeBSD versions.

Add a patch taken from upstream. Tested successfully on 11.3-RELEASE amd64.

The upstream fix is an extended version of a fix proposed by
Dave Mueller in the PR.

See also:
http://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2019q4/013412.html

PR: 241068
Reported by: Phil Chadwick
Obtained from: Simon Kelley, http://thekelleys.org.uk/gitweb/?p=dnsmasq.git;a=commit;h=936bd82755e8f75fc09c1e9a67fb390175b157d4
MFH: 2019Q4