When a line only contains non-range atoms we can end-up accessing
past the end atom.
Add a test that shows the issue when run with valgrind, it is
unfortunately quite hard to trigger a crash because the invalidly
accessed byte usually leads to the correct code path being taken
(when != DisplayAtom::Range) so we have only 1 in 255 chance of
triggerring a crash.
Fixes#4927
