Staging: batman-adv: Remove NULL pointer check

It is safe to call kfree(NULL) which makes this extra check unneeded. It
was found using checkpatch.pl from linux-2.6

Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Acked-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>