alloc_wrb_handle already does memset zero of iscsi_wrb descriptor so
remove redundant memset in WRB submission paths.

Add pwrb_handle NULL check before memsett'ing pwrb.
Signed-off-by: Jitendra Bhivare <jitendra.bhivare@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>