REPLACE should throw an error when given negative indices

[T.Y Chew]

(let ((x (make-array 10 :initial-contents "abcdefghij"))

(y (make-array 10 :initial-contents "0123456789")))

(replace x y :start2 0 :end2 -8)
(list x y))

==>
(#(#\0 #\1 #\2 #\3 #\4 #\5 #\6 #\7 #\8 #\9)

#(#\0 #\1 #\2 #\3 #\4 #\5 #\6 #\7 #\8 #\9))

The error messages (when we get them) could be quite confusing:

(let ((x (make-array 10 :initial-contents "abcdefghij"))

(y (make-array 10 :initial-contents "0123456789")))

(replace x y :start1 0 :end1 -1 :start2 0 :end2 -1)
(list x y))

signals:

Invalid array index 10 for #(0 1 2 3 4 5 6 7 8 9) (should be >= 0 and < 10).

[Condition of type TYPE-ERROR]