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)))]

gui-test/tests/gracket/windowing.rktl

@@ -875,7 +875,9 @@
                    (st '(0 1) l get-column-order)
                    (stv l set-column-order '(1 0))
                    (st '(1 0) l get-column-order)
+                   (define old (send l get-string 0))
                    (stv l set-string 0 "A2" 1)
+                   (st old l get-string 0)
                    (check-col-width 1)
                    (stv l append-column "Three")
                    (check-col-width 2)