l2tpv3: fix possible double free

freeaddrinfo(result) does not assign result = NULL, after frees it.
There will be a double free when it goes error case.
It is reported by covertiy.

Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
