Discovered this while running iSCSI.iSCSITMF AbortTaskSimpleAsync
test case. For Task Management command iser_pdu->iscsi_pdu.scsi_cbdata
is not set. When test case tries to send Task Management command
via common API iser_send_command() - it calls overflow_data_size
which tries to dereference scsi_cbdata leading to SEGFAULT.
Added a non-NULL check for scsi_cbdata before accessing it.