don't set list-box% content when set-string given column > 0

  - calling set-string with a column argument other than 0 overwrites
  content, which doesn't appear to store any data other than the
  string to display in column 0. this breaks future calls to
  get-string.

Author: jjsimpso

gui-lib/mred/private/mritem.rkt

@@ -903,7 +903,8 @@
                                            (sub1 num-columns)
                                            #f)))
                     (check-item 'set-string n)
-                    (send this -set-list-string n d)
+                    (when (= col 0)
+                      (send this -set-list-string n d))
                     (send wx set-string n d col)))]
      [set-data (entry-point (lambda (n d) (check-item 'set-data n) (send wx set-data n d)))]
      [get-first-visible-item (entry-point (lambda () (send wx get-first-item)))]