fix optimizer bug

Closes #1461

Thanks to Gustavo for tracking down the problem.

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

@@ -6230,6 +6230,16 @@
     (eval (read (open-input-bytes (get-output-bytes o)))))
   ((dynamic-require ''uses-mutator-with-an-auto-field 'f) #f))
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Make sure that the optimizer doesn't discard a known error on the
+;; right-hand side of a `letrec`
+
+(err/rt-test
+ (letrec-values ([() (list (3) the-val)]
+                 [(the-val) 42])
+                777)
+ exn:fail:contract?)
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

racket/src/racket/src/optimize.c

@@ -7136,7 +7136,7 @@ static Scheme_Object *optimize_lets(Scheme_Object *form, Optimize_Info *info, in
        (values (if id e1 e2) ...) and then split the values call, since
        duplicating the id use and test is likely to pay off. */
     if ((pre_body->count != 1)
-        && (found_escapes
+        && ((!is_rec && found_escapes)
             || (is_values_apply(value, pre_body->count, rhs_info, merge_skip_vars, 1)
                 && ((!is_rec && no_mutable_bindings(pre_body))
                     /* If the right-hand side is omittable, then there are