batman-adv: check return type of unicast packet preparations

batadv_unicast(_4addr)_prepare_skb  might reallocate the skb's data.
And if it tries to do so then this can potentially fail.

We shouldn't continue working on this skb in such a case.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Acked-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>