fix syntax checking in for/vector

Closes #1308

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

@@ -549,6 +549,8 @@
 ;; for/fold syntax checking
 (syntax-test #'(for/fold () bad 1) #rx".*bad sequence binding clauses.*")
 
+(syntax-test #'(for/vector ()) #rx".*missing body.*")
+
 ;; specific hash set iterators
 (err/rt-test (for/sum ([x (in-immutable-set '(1 2))]) x)
              exn:fail:contract?

racket/collects/racket/private/for.rkt

@@ -1534,10 +1534,14 @@
             (cond
              [(null? exprs)
               (if (null? post-kw)
+                  (if (null? pre-kw)
+                      (raise-syntax-error #f
+                                          "missing body"
+                                          stx)
                       (raise-syntax-error #f
                                           (format "missing body form after ~a clause" (syntax-e (cadr pre-kw)))
                                           stx
-                                      (cadr pre-kw))
+                                          (cadr pre-kw)))
                   (list (reverse pre-kw) (reverse post-kw)))]
              [(memq (syntax-e (car exprs)) '(#:break #:final))
               (if (pair? (cdr exprs))