MINOR: ssl: Don't forget to call the close method of the underlying xprt.

MINOR: ssl: Don't forget to call the close method of the underlying xprt.

In ssl_sock_close(), don't forget to call the underlying xprt's close method
if it exists. For now it's harmless not to do so, because the only available
layer is the raw socket, which doesn't have a close method, but that will
change when we implement QUIC.