batman-adv: Split a condition check

Let us split a check for a condition at the beginning of the
batadv_is_ap_isolated() function so that a direct return can be performed
in this function if the variable "vlan" contained a null pointer.

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>