clone: use xfopen() instead of fopen()

copy_alternates() called fopen() without handling errors. By switching
to xfopen(), this bug is fixed.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/clone.c

@@ -357,7 +357,7 @@ static void copy_alternates(struct strbuf *src, struct strbuf *dst,
 	 * to turn entries with paths relative to the original
 	 * absolute, so that they can be used in the new repository.
 	 */
-	FILE *in = fopen(src->buf, "r");
+	FILE *in = xfopen(src->buf, "r");
 	struct strbuf line = STRBUF_INIT;
 
 	while (strbuf_getline(&line, in) != EOF) {