ensure that window size information is always initialized

Fixes a divide-by-zero crash reported by sthen@. To reproduce:

  printf 'show\nshow run\n' > /tmp/nsh.script
  nsh -i /tmp/nsh.script
  
ok chris

main.c

@@ -255,6 +255,8 @@ main(int argc, char *argv[])
 		 */
 		priv = 1;
 
+		setwinsize(0);
+
 		create_db();
 
 		/*
@@ -277,6 +279,8 @@ main(int argc, char *argv[])
 		 */
 		priv = 1;
 
+		setwinsize(0);
+
 		create_db();
 
 		cmdrc(rc);