fix `image-snip%' unmarshaling with filename

Merge to 5.1
(cherry picked from commit c2c6c79a15)

collects/racket/snip/private/snip.rkt

@@ -787,8 +787,7 @@
   (def/override (read [editor-stream-in% f])
     (let ([scl (get-the-snip-class-list)]
           [can-inline? ((send f do-reading-version this) . > . 1)])
-      (let ([filename (let ([s (send f get-bytes #f)])
-                        (subbytes s 0 (max 0 (sub1 (bytes-length s)))))])
+      (let ([filename (send f get-bytes #f)])
         (let-boxes ([type 0]
                     [w 0.0]
                     [h 0.0]

collects/tests/gracket/wxme.rkt

@@ -1347,4 +1347,23 @@
 
 ;; ----------------------------------------
 
+(let ()
+  (define (mk) (make-object image-snip% (collection-file-path "b-run.png" "icons") 'unknown #f #f))
+  
+  (define is (mk))
+  (define copy-is
+    (let ()
+      (define sp (open-output-string))
+      (define t (new text%))
+      (send t insert (mk))
+      (send t save-port sp)
+      (define t2 (new text%))
+      (send t2 insert-port (open-input-string (get-output-string sp)))
+      (send t2 find-first-snip)))
+  
+  (expect (send (mk) get-filename)
+          (send copy-is get-filename)))
+
+;; ----------------------------------------
+
 (done)