Calling send() or write() on a connected UDP socket raises a
kernel panic in posix_sockets.c, line 215, because _sockaddr_to_ep()
is called from socket_sendto() with address set to NULL.
This change modifies socket_sendto() to
* Check if the UDP socket is connected when the address argument
is NULL.
* If connected, use the stored remote endpoint for sock_udp_send().
83e2932eb6