Incorrect sequence of operations with DCB

Partly because of MXS-196 there is a possibility that a DCB fails to start polling in the call to func.listen around line 351 of service.c but the DCB will be in state DCB_STATE_POLLING. When dcb_close is called there will be an attempt to remove the DCB from the poll list, but since it was never successfully added this will fail. The dcb_close function will emit an error, but not otherwise do anything, leaving a DCB in limbo.