batman-adv: Even Batman should not dereference NULL pointers

There's a problem in net/batman-adv/unicast.c::frag_send_skb().
dev_alloc_skb() allocates memory and may fail, thus returning NULL. If
this happens we'll pass a NULL pointer on to skb_split() which in turn
hands it to skb_split_inside_header() from where it gets passed to
skb_put() that lets skb_tail_pointer() play with it and that function
dereferences it. And thus the bat dies.

While I was at it I also moved the call to dev_alloc_skb() above the
assignment to 'unicast_packet' since there's no reason to do that
assignment if the memory allocation fails.
Signed-off-by: NJesper Juhl <jj@chaosbits.net>
Signed-off-by: NSven Eckelmann <sven@narfation.org>

net/batman-adv/unicast.c

@@ -229,10 +229,12 @@ int frag_send_skb(struct sk_buff *skb, struct bat_priv *bat_priv,
 	if (!bat_priv->primary_if)
 		goto dropped;
 
-	unicast_packet = (struct unicast_packet *) skb->data;
+	frag_skb = dev_alloc_skb(data_len - (data_len / 2) + ucf_hdr_len);
+	if (!frag_skb)
+		goto dropped;
 
+	unicast_packet = (struct unicast_packet *) skb->data;
 	memcpy(&tmp_uc, unicast_packet, uc_hdr_len);
-	frag_skb = dev_alloc_skb(data_len - (data_len / 2) + ucf_hdr_len);
 	skb_split(skb, frag_skb, data_len / 2);
 
 	if (my_skb_head_push(skb, ucf_hdr_len - uc_hdr_len) < 0 ||