mafft hangs at 100% CPU for over a week

[Tony Travis]

The "mafft" program hangs waiting for "disttbfast" to exit, but in some cases "disttbfast" does not exit after many days. This problem only seems to occur with a particular alignment of virus sequences:

/usr/bin/mafft --inputorder --adjustdirection --thread 4 --auto phygo.Spbetalikevirus.backbone.seqs

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: mafft 7.453-1
ProcVersionSignature: Ubuntu 5.4.0-45.49-generic 5.4.55
Uname: Linux 5.4.0-45-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27.8
Architecture: amd64
CasperMD5CheckResult: skip
Date: Mon Sep 28 13:28:52 2020
InstallationDate: Installed on 2020-06-15 (104 days ago)
InstallationMedia: Ubuntu-MATE 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
SourcePackage: mafft
UpgradeStatus: No upgrade log present (probably fresh install)