batman-adv: dat: handle forward allocation error

In the Linux kernel, the following vulnerability has been resolved:

batman-adv: dat: handle forward allocation error

batadv_dat_forward_data() calls pskb_copy_for_clone() to duplicate an skb

for each DHT candidate, but does not check the return value before passing

it to batadv_send_skb_prepare_unicast_4addr(). That function dereferences

the skb unconditionally, so a failed allocation triggers a NULL pointer

dereference.

Skip forwarding to the current DHT candidate on allocation failure.