Subject: bug#19324: 25.0.50; add-function and nil

From: Leo Liu <[email-masked]>
Date: Tue, 09 Dec 2014 12:55:22 +0800

1. Goto a buffer (e.g. text-mode) with eldoc-documentation-function nil
2. (add-function :after-until (local 'eldoc-documentation-function) #'ignore)
3. Move around to trigger eldoc error: (void-function nil)

Looks like the orig function is unconditionally called without checking
it's a function object.

Leo