fixed unfold in list.scm

[nortiero]

Hello!

Minimal patch to unfold, still missing additional checks present in the reference.

These one-liners now work:

(unfold null-list? car cdr '(1 2 3 4 5) )                            
=> (1 2 3 4 5)

(unfold null-list? car cdr '(1 2 3 4 5) (lambda (x) 'this-is-the-end))  
=> (1 2 3 4 5 . this-is-the-end)

[alvatar]

Thank you @nortiero ! Merging.