nutdrv_qx: prevent a vicious loop when unexpected answers happen

If a 'QX_FLAG_QUICK_POLL' item gets an unexpected (non-empty) answer and, after returning from 'qx_ups_walk()', it is not followed by at least one item using a different 'command', the driver will loop endlessly using the same 'broken' answer instead of trying to get a new one from the UPS.
To solve this issue, make sure to have an empty 'previous_item' when starting 'qx_ups_walk()'.
Also, bail out of 'qx_ups_walk()' when a 'QX_FLAG_QUICK_POLL' item can't be preprocessed properly through 'ups_infoval_set()'.