Use a fresh cons for %stream-null.

* module/srfi/srfi-41.scm (%stream-null): Use a fresh cons rather than
  a literal cons. You never know what peval constant-folding could do
  with the latter, either now or in the future.

module/srfi/srfi-41.scm

@@ -148,7 +148,7 @@
 
 (define stream? stream-promise?)
 
-(define %stream-null '(stream . null))
+(define %stream-null (cons 'stream 'null))
 (define stream-null (stream-eager %stream-null))
 
 (define (stream-null? obj)