TLS: check for sock->recv_cb when handling received data

TLS: check for sock->recv_cb when handling received data

This commit adds a check if 'sock->recv_cb' might have been nullified
during the call to 'sock->recv_cb'. That could happen, e.g. by an
indirect call to 'isc_nmhandle_close()' from within the callback when
wrapping up.

In this case, let's close the TLS connection.