nbd: fix memory leak on socket_connect failed

When TCP connection fails between nbd server and client,
the local var, sioc, memory leak.

This patch fixes the memory leak.

Signed-off-by: yaolujing <yaolujing@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1491005709-29989-1-git-send-email-yaolujing@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
