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>

net/batman-adv/main.c

@@ -475,7 +475,7 @@ static int batadv_param_set_ra(const char *val, const struct kernel_param *kp)
 	char *algo_name = (char *)val;
 	size_t name_len = strlen(algo_name);
 
-	if (algo_name[name_len - 1] == '\n')
+	if (name_len > 0 && algo_name[name_len - 1] == '\n')
 		algo_name[name_len - 1] = '\0';
 
 	bat_algo_ops = batadv_algo_get(algo_name);