batman-adv: check proto length before accessing proto string buffer

batadv_param_set_ra() strips the trailing '\n' from the supplied
string buffer without checking the length of the buffer first. This
patches avoids random memory access and associated potential
crashes.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>