Set the right variable to NULL when unsetting ipv6_gateway

We were freeing one and setting a different one to NULL, eventually
leading to a crash when closing the netdev (at container shutdown)
and freeing already-freed memory.

Closes #732

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>

src/lxc/confile.c

@@ -971,7 +971,7 @@ static int config_network_ipv6_gateway(const char *key, const char *value,
 	free(netdev->ipv6_gateway);
 
 	if (!value || strlen(value) == 0) {
-		netdev->ipv4_gateway = NULL;
+		netdev->ipv6_gateway = NULL;
 	} else if (!strcmp(value, "auto")) {
 		netdev->ipv6_gateway = NULL;
 		netdev->ipv6_gateway_auto = true;