|reopen| #17062


Unfortunatly can't deliver a backtrace, as it was some times ago.

The bug is visible by program-logic already.

fill-match-adaptive-prefix counts on current-fill-column having:

(>= (+ (current-left-margin) (length str)) (current-fill-column))

This will be broken if current-fill-column returns nil.

Returning nil is possible, see inside current-fill-column:

     (if fill-column

If fill-column is nil, current-fill-column will return nil which was the 
case coming upon.
A fix might make sure an integer is returned anyway: think at 0 or 
default value.