fix ad-hoc hash-ref optimizaiton

Optimization to convert `(hash-ref <ht> <key> (lambda () <constant>))`
to `(hash-ref <ht> <key> <constant>)` didn't check that the `lambda`
for had zero argument.

Closes #1648

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

@@ -3198,6 +3198,26 @@
               (hash-ref '#hash((x . y)) x add1))
            #f)
 
+(test-comp '(lambda ()
+             (hash-ref #hash()
+                       'missing
+                       (λ ()
+                         'UNEXPECTED!)))
+           '(lambda ()
+             (hash-ref #hash()
+                       'missing
+                       'UNEXPECTED!)))
+(test-comp '(lambda ()
+             (hash-ref #hash()
+                       'missing
+                       (λ (required-arg)
+                         'UNEXPECTED!)))
+           '(lambda ()
+             (hash-ref #hash()
+                       'missing
+                       'UNEXPECTED!))
+           #f)
+
 ;; Check elimination of ignored structure predicate
 ;; and constructor applications:
 

racket/src/racket/src/optimize.c

@@ -3812,6 +3812,7 @@ static Scheme_Object *optimize_application(Scheme_Object *o, Optimize_Info *info
       && SAME_OBJ(scheme_hash_ref_proc, app->args[0])
       && SCHEME_HASHTRP(app->args[1])
       && SAME_TYPE(scheme_ir_lambda_type, SCHEME_TYPE(app->args[3]))
+      && (((Scheme_Lambda *)(app->args[3]))->num_params == 0)
       && (SCHEME_TYPE(((Scheme_Lambda *)app->args[3])->body) > _scheme_ir_values_types_)
       && !SCHEME_PROCP(((Scheme_Lambda *)app->args[3])->body)) {
     app->args[3] = ((Scheme_Lambda *)app->args[3])->body;