sock: tcp: fix sock_tcp_read() example
sock: tcp: fix sock_tcp_read() example

sock_tcp_read() will return 0 if the connection was closed, we
need to check for this.

miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kYc0o/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,miri64/RIOT,miri64/RIOT,ant9000/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,jasonatran/RIOT,OlegHahm/RIOT,miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,kYc0o/RIOT,OlegHahm/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,ant9000/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,OlegHahm/RIOT