fix wcm on error in let's

Repairs a problem with 8190a7730d, which can incorerctly
move an erroring experssion into tail position.

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

@@ -3893,6 +3893,20 @@
 (test-comp '(letrec-values ([(x y) (error "oops")]) 11)
            '(error "oops"))
 
+(test-comp '(with-continuation-mark
+             'x 'y
+             (let-values ([() (with-continuation-mark
+                                  'x 'z
+                                  (error "oops"))])
+               11))
+           '(with-continuation-mark
+             'x 'y
+             (begin0
+              (with-continuation-mark
+                  'x 'z
+                  (error "oops"))
+              (void))))
+
 (test-comp `(module m racket/base
               (define x 5)
               (set! x 3)

racket/src/racket/src/optimize.c

@@ -7850,7 +7850,7 @@ static Scheme_Object *optimize_lets(Scheme_Object *form, Optimize_Info *info, in
   if (!found_escapes) {
     body = scheme_optimize_expr(body, body_info, scheme_optimize_tail_context(context));
   } else {
-    body = escape_body;
+    body = ensure_noncm(escape_body);
     body_info->single_result = 1;
     body_info->preserves_marks = 1;
     body_info->escapes = 1;