batman-adv: dat: handle forward allocation error

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.

Cc: stable@kernel.org
Fixes: 785ea1144182 ("batman-adv: Distributed ARP Table - create DHT helper functions")
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Reviewed-by: Yuan Tan <yuantan098@gmail.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>