batman-adv: add sanity check when removing global tts

After removing the batman-adv module, the hash may be already gone
when tt_global_del_orig() tries to clean the hash. This patch adds
a sanity check to avoid this.
Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Tested-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>

net/batman-adv/translation-table.c

@@ -698,6 +698,9 @@ void tt_global_del_orig(struct bat_priv *bat_priv,
 	struct hlist_head *head;
 	spinlock_t *list_lock; /* protects write access to the hash lists */
 
+	if (!hash)
+		return;
+
 	for (i = 0; i < hash->size; i++) {
 		head = &hash->table[i];
 		list_lock = &hash->list_locks[i];