A rare crash in the DoH code caused by an assert

A rare issue was found while working on !5309 (merged). The unit test suite (doh_test) revealed a situation when session->handle got detached too early in the http_send_outgoing(), the function which takes data from nghttp2 and sends it via the underlying connection.

As a result, when we have reached the call to isc_nm_send()

	session->sending++;
	isc_nm_send(session->handle, &send->data, http_writecb, send);
 	return (true);
}

the session->handle was NULL triggering an assert:

void
isc_nm_send(isc_nmhandle_t *handle, isc_region_t *region, isc_nm_cb_t cb,
	    void *cbarg) {
	REQUIRE(VALID_NMHANDLE(handle));