util: fix another cunescape() regression

util: fix another cunescape() regression

Fix a regression caused by 4034a06d ("util: rework word parsing and c
unescaping code") which broke octal escape sequences.

The reason for this breakage is that cunescape_one() expects 4 characters
in an octal encoding, which is a stray left-over from the old code which
operated on different variables to make the length check.

While at it, add a test case to prevent the same thing from happening
again.