batman-adv: properly check pskb_may_pull return value

pskb_may_pull() returns 1 on success and 0 in case of failure,
therefore checking for the return value being negative does
not make sense at all.

This way if the function fails we will probably read beyond the current
skb data buffer. Fix this by doing the proper check.

Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>