batman-adv: Less checks in batadv_tvlv_unicast_send()

* Let us return directly if a call of the batadv_orig_hash_find() function
  returned a null pointer.

* Omit the initialisation for the variable "skb" at the beginning.

* Replace an assignment by a call of the kfree_skb() function
  and delete the affected variable "ret" then.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>