Ticket #2663: Segmentation fault while background copying

Action to reproduce bug:
Start copy/move file/files (around 100mb+) in background

Signed-off-by: Slava Zanko <slavazanko@gmail.com>

src/main.c

@@ -319,7 +319,7 @@ do_load_prompt (void)
         return ret;
 
     /* Don't actually change the prompt if it's invisible */
-    if (((Dlg_head *) top_dlg->data == midnight_dlg) && command_prompt)
+    if (top_dlg != NULL && ((Dlg_head *) top_dlg->data == midnight_dlg) && command_prompt)
     {
         setup_cmdline ();