net/rdesktop: update 1.8.3 -> 1.8.4

• many more CVEs are fixed by this upgrade, see Relnotes

PR: 235885, 229029
Submitted by: Greg Veldman <freebsd@gregv.net>
Reviewed by: w.schwarzenfeld@utanet.at, brnd, cem, joneum
Approved by: gregf@hugops.pw (maintainer timeout)
Relnotes: https://github.com/rdesktop/rdesktop/releases/tag/v1.8.4
Security: CVE-2018-8794
MFH: 2019Q1
Differential Revision: https://reviews.freebsd.org/D18566