Heap corruption & crash with -Tterm

[paravoid]

With current master (0.7.7):

$ cat /tmp/repro.md
# NAME

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
$ ./lowdown -Tterm /tmp/repro.md
free(): corrupted unsorted chunks
Aborted

git bisect shows fb44bef (Make -Tterm output aware of UTF-8 characters when computing column width.) as the first bad commit.

valgrind is showing a bunch of errors in the same codepaths, and none with the commit above reverted. Happy to provide the valgrind output, if that helps.