fix another problem with attempted overflow clean-up

collects/tests/racket/basic.rktl

@@ -657,6 +657,7 @@
 (unless 64-bit-machine?
   (err/rt-test (make-vector 1234567890 #\f) exn:fail:out-of-memory?)
   (err/rt-test (read (open-input-string "#1234567890(0)")) exn:fail:out-of-memory?))
+(test #t vector? (make-vector 0))
 
 (define f (make-string 3 #\*))
 (test "?**" 'string-set! (begin (string-set! f 0 #\?) f))

src/racket/src/vector.c

@@ -235,7 +235,7 @@ scheme_init_unsafe_vector (Scheme_Env *env)
 }
 
 #define VECTOR_BYTES(size) (sizeof(Scheme_Vector) + ((size) - 1) * sizeof(Scheme_Object *))
-#define REV_VECTOR_BYTES(size) (((size - sizeof(Scheme_Vector)) / sizeof(Scheme_Object *)) + 1)
+#define REV_VECTOR_BYTES(size) (((size) - (sizeof(Scheme_Vector) - sizeof(Scheme_Object *))) / sizeof(Scheme_Object *))
 
 Scheme_Object *
 scheme_make_vector (intptr_t size, Scheme_Object *fill)