[a11y] Fix assertion failure
[a11y] Fix assertion failure

Reported here:
https://bugs.launchpad.net/ubuntu/+source/gnome-terminal/+bug/435646

thestinger/vte-ng,aligo/vte,thestinger/vte-ng,asivokon/vte-fx,jessevdk/vte,flaxandteal/gasket-vte,asivokon/vte-fx,pjm0616/vte,asivokon/vte-fx,rvu95/vte,gloob/vte-copy,aligo/vte,jessevdk/vte,flaxandteal/gasket-vte,asivokon/vte-fx,aligo/vte,saitoha/vte-sixel,jessevdk/vte,saitoha/vte-sixel,pjm0616/vte,saitoha/vte-sixel,gloob/vte-copy,rvu95/vte,flaxandteal/gasket-vte,flaxandteal/gasket-vte,rvu95/vte,thestinger/vte-ng,gloob/vte-copy,pjm0616/vte,jessevdk/vte,rvu95/vte