net/socket: psock_send/psock_sendto: remove assert check for null psock and...

net/socket: psock_send/psock_sendto: remove assert check for null psock and buf input pointers.  Removes check as 'psock == NULL' altogether because that checked for later in psock_send and psock_sendto. Change null check for 'buf' so that it is handled same as in recvfrom.c (return -EINVAL instead of
assert).