Swap remaining unsafe and chaperone-unsafe operation in the TR optimizer.

Fixes the fix in aba046a92d.

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/optimizer/vector.rkt

@@ -75,7 +75,7 @@
                 [new-v v.opt])
             ;; do the impersonator check up front, to avoid doing it twice (length and op)
             (if (impersonator? new-v)
-                (if #,(let ([one-sided #'(unsafe-fx< new-i (unsafe-vector*-length new-v))])
+                (if #,(let ([one-sided #'(unsafe-fx< new-i (unsafe-vector-length new-v))])
                         (if i-known-nonneg?
                             ;; we know it's nonnegative, one-sided check
                             one-sided
@@ -84,7 +84,7 @@
                     (op.unsafe new-v new-i new.opt ...)
                     #,safe-fallback) ; will error. to give the right error message
                 ;; not an impersonator, can use unsafe-vector* ops
-                (if #,(let ([one-sided #'(unsafe-fx< new-i (unsafe-vector-length new-v))])
+                (if #,(let ([one-sided #'(unsafe-fx< new-i (unsafe-vector*-length new-v))])
                         (if i-known-nonneg?
                             one-sided
                             #`(and (unsafe-fx>= new-i 0)

pkgs/typed-racket-pkgs/typed-racket-test/tests/typed-racket/optimizer/tests/vector-length-impersonator.rkt

@@ -0,0 +1,23 @@
+#;#;
+#<<END
+END
+#<<END
+passed
+
+END
+
+#lang typed/racket
+
+(: x (Vectorof Number))
+(define x (make-vector 0 0))
+(: y (Vectorof Float))
+(define z (cast x '#()))
+(define y (cast z (Vectorof Float)))
+
+;; should error
+(with-handlers ([exn:fail:contract?
+                 (lambda (e)
+                   (when (regexp-match "index is out of range for empty vector"
+                                       (exn-message e))
+                     (display "passed\n")))])
+  (vector-set! y 1 3.0))