nanocoap_sock: release rx buffer on RESET
nanocoap_sock: release rx buffer on RESET

We must not return without calling sock_udp_recv_buf() again.

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