Put back an accidentally elided ELSE branch in PLACEHOLDER-SET!

scheme/big/placeholder.scm

@@ -39,10 +39,11 @@
 				 => (lambda (thread)
 				      (loop (cons thread waiters))))
 				(else
-				 waiters))))))))))
+				 waiters))))
+			    (else #f)))))))
     (if waiters
 	(for-each make-ready waiters)
 	(if (not (eq? value (placeholder-value placeholder)))
 	    (error "placeholder is already assigned"
 		   placeholder
-		   value)))))
+		   value)))))