tdb2: restore file filling code.

This snuck in fe55330a which added the stats attribute.  Without it,
TDB works but is vulnerable to segmenation faults or write errors when
disk is exhausted.

ccan/tdb2/io.c

@@ -404,7 +404,7 @@ static int tdb_expand_file(struct tdb_context *tdb, tdb_len_t addition)
 		   file isn't sparse, which would be very bad if we ran out of
 		   disk. This must be done with write, not via mmap */
 		memset(buf, 0x43, sizeof(buf));
-		if (0 || fill(tdb, buf, sizeof(buf), tdb->map_size, addition) == -1)
+		if (fill(tdb, buf, sizeof(buf), tdb->map_size, addition) == -1)
 			return -1;
 		tdb->map_size += addition;
 		tdb_mmap(tdb);