fix memory-management bug in struct->vector

Closes PR 15410

pkgs/racket-test-core/tests/racket/chaperone.rktl

@@ -2583,6 +2583,19 @@
   (struct b a (z w))
   (test #t impersonator? (impersonate-struct (b 1 2 3 4) set-a-y! void)))
 
+;; ----------------------------------------
+;; Make sure `struct->vector` doesnt' have memory-management issues
+;; with impersonated structs:
+
+(let ()
+  (struct s (x y z) #:mutable #:transparent)
+  (test #(struct:s 1 2 3)
+        struct->vector
+        (impersonate-struct (s 1 2 3)
+                            s-x (lambda (s v) (collect-garbage 'minor) v)
+                            s-y (lambda (s v) (collect-garbage 'minor) v)
+                            s-z (lambda (s v) (collect-garbage 'minor) v))))
+
 ;; ----------------------------------------
 
 (report-errs)

racket/src/racket/src/struct.c

@@ -3302,8 +3302,12 @@ Scheme_Object *scheme_struct_to_vector(Scheme_Object *_s, Scheme_Object *unknown
         --i;
         if (SAME_OBJ((Scheme_Object *)s, _s))
           elem = s->slots[i];
-        else
+        else {
+          /* scheme_struct_ref() may trigger a GC; see "BEWARE" above */
+          array = NULL;
           elem = scheme_struct_ref(_s, i);
+          array = SCHEME_VEC_ELS(v);
+        }
 	array[1 + (--m)] = elem;
       }
     }