DRV: Fixed missing pointer reference crash when ACK!=OK.
DRV: Fixed missing pointer reference crash when ACK!=OK.

As ACK!=OK may require a data phase and LibSWD does this data phase always
no matter if sticky error is enabled or not, there were situations where
truncated tail contained non-existent elements that were later referenced.

cedar-renjun/LibSWD,cedar-renjun/LibSWD,cedar-renjun/LibSWD,ongjohn/LibSWD,ongjohn/LibSWD,shangdawei/LibSWD,ongjohn/LibSWD,shangdawei/LibSWD,shangdawei/LibSWD