rl_redisplay_function null situation

[Anon Anonson]

I would like to re-report the following bug / documentation error: 
https://mail.gnu.org/archive/html/bug-readline/2013-09/msg00021.html

The original writer forgot to mention that the documentation explicitly 
says:

> If non-zero, [...]

This should be a minor fix, I will gladly patch it myself, however the 
one in authority should decide what the correct behaviour is.