detect whitespace issues in rfc/index.txt earlier

by checking with each fetch and update.

rfc/fetch.sh

@@ -1,4 +1,13 @@
-#!/bin/sh
+#!/usr/bin/env bash
+set -uo pipefail
+
+if grep '	' index.txt  | grep -E -v '^[^	 ]+	[^	 ]+	[^	 ]+	[^	]+$$'; then
+	echo 'bad lines in rfc/index.txt'
+	exit 1
+fi
+
+set -e
+
 for number in $(sed -n 's/^\([0-9][0-9]*\)[ \t].*$/\1/p' index.txt); do
 	if ! test -f "$number"; then
 		curl https://www.rfc-editor.org/rfc/rfc$number.txt >$number || rm $number