grep: handle corrupt index files early

Any other caller of 'repo_read_index' dies upon a negative return of
it, so grep should, too.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -480,7 +480,8 @@ static int grep_cache(struct grep_opt *opt, struct repository *repo,
 		strbuf_addstr(&name, repo->submodule_prefix);
 	}
 
-	repo_read_index(repo);
+	if (repo_read_index(repo) < 0)
+		die("index file corrupt");
 
 	for (nr = 0; nr < repo->index->cache_nr; nr++) {
 		const struct cache_entry *ce = repo->index->cache[nr];