Correct handling of procedure env roots in mutator

collects/plai/mutator.rkt

@@ -178,8 +178,8 @@
          (quasisyntax/loc stx
            (let ([closure (lambda (id ...) 
                             (syntax-parameterize ([mutator-env-roots 
-                                                   (list* #'id ...
-                                                          (syntax-parameter-value #'mutator-env-roots))]
+                                                   (list #'id ...
+                                                         #'free-id ...)]
                                                   [mutator-tail-call? #t])
                                                  (->address body)))])
              (add-closure-env! closure (list (make-env-root free-id) ...))

collects/tests/plai/gc/good-mutators/danny-bug.rkt

@@ -0,0 +1,12 @@
+#lang plai/mutator
+(allocator-setup "../good-collectors/good-collector.rkt" 6)
+
+(define
+  proc
+  (let* ([not-root 1] ; 2
+         [root 2]) ; 4
+    (lambda () ; 6
+      3
+      root)))
+
+(proc)