avoid error messages when finally closing the display device.

these messages creeped in with commit
  2012-08-19 18:58:31
  "plug memory leak in x11.c"

when the "printf" device Close and Update
 functions might be invoked on program termination.

use the silent "nop" implementation
instead of the error reporting "nodev" implementation

src/frontend/display.c

@@ -121,10 +121,10 @@ DISPDEVICE device[] = {
 
     { "printf", 0, 0, 24, 80, 0, 0,
       (disp_fn_Init_t *) nodev, (disp_fn_NewViewport_t *)  nodev,
-      (disp_fn_Close_t *) nodev, (disp_fn_Clear_t *) nodev,
+      (disp_fn_Close_t *) nop, (disp_fn_Clear_t *) nodev,
       (disp_fn_DrawLine_t *) nodev, (disp_fn_Arc_t *) nodev, (disp_fn_Text_t *) nodev,
       (disp_fn_DefineColor_t *) nodev, (disp_fn_DefineLinestyle_t *) nodev,
-      (disp_fn_SetLinestyle_t *) nodev, (disp_fn_SetColor_t *) nodev, (disp_fn_Update_t *) nodev,
+      (disp_fn_SetLinestyle_t *) nodev, (disp_fn_SetColor_t *) nodev, (disp_fn_Update_t *) nop,
       (disp_fn_Track_t *) nodev, (disp_fn_MakeMenu_t *) nodev, (disp_fn_MakeDialog_t *) nodev, gen_Input,
       (disp_fn_DatatoScreen_t *) nodev, },