scsi: libcxgbi: add check for valid cxgbi_task_data

In error case it is possible that ->cleanup_task() gets called without
calling ->alloc_pdu() in this case cxgbi_task_data is not valid, so add
a check for for valid cxgbi_task_data in cxgbi_cleanup_task().

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>