zpool: things get messy with 0 columns

zpool iostat with terminal reporting 0 columns will print out
a very long line. Use columns 80 when query reports 0.

Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Toomas Soome <[email protected]>
Closes #19051

cmd/zpool/zpool_main.c

@@ -5872,7 +5872,7 @@ get_columns(void)
 
 	if (isatty(STDOUT_FILENO)) {
 		error = ioctl(STDOUT_FILENO, TIOCGWINSZ, &ws);
-		if (error == 0)
+		if (error == 0 && ws.ws_col > 0)
 			columns = ws.ws_col;
 	} else {
 		columns = 999;