vis: handle file truncation more gracefully

If we use the file / virtual memory system as cache (using mmap(2))
and another process truncates the file we are editing, we have a
problem. All we can do is catch the resulting SIGBUS, close the
corresponding window and print a warning message.

To test this use:

 $ dd if=/dev/zero of=TEST bs=8M count=1
 $ vis TEST
   :! echo TRUNCATE > TEST

1 files changed, 2 insertions, 0 deletions

diff --git a/editor.c b/editor.c
index ad9bd4e..8628458 100644
--- a/editor.c
+++ b/editor.c
@@ -358,6 +358,8 @@ void editor_window_close(Win *win) {
 		ed->windows = win->next;
 	if (ed->win == win)
 		ed->win = win->next ? win->next : win->prev;
+	if (ed->prompt_window == win)
+		ed->prompt_window = NULL;
 	window_free(win);
 	if (ed->win)
 		ed->ui->window_focus(ed->win->ui);