Rewrite (take n x) when n is more than (len x).
This rule may sometimes lead your proof in a bad direction. If you see unwanted repeat terms, you may want to disable it.
Theorem: take-of-too-many
(defthm take-of-too-many (implies (<= (len x) (nfix n)) (equal (take n x) (append x (repeat (- (nfix n) (len x)) nil)))))