Insufficient error handling in httpd.c

Function httpd_accept in httpd.c does not write any error logs if things fail. In branch MXS-329 I have altered line 363 to:

if (NULL == client->session || poll_add_dcb(client) == -1)

which adds a check to handle failure of the session_alloc call.

However, it would appear that the DCB that was created on line 348 is still hanging around to no purpose and probably needs to be sent to dcb_close().