scsi: qla2xxx: Make qlt_handle_abts_completion() more robust

Avoid that this function crashes if mcmd == NULL.

Cc: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: NBart Van Assche <bvanassche@acm.org>
Tested-by: NHimanshu Madhani <hmadhani@marvell.com>
Reviewed-by: NHimanshu Madhani <hmadhani@marvell.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/qla2xxx/qla_target.c

@@ -5731,7 +5731,7 @@ static void qlt_handle_abts_completion(struct scsi_qla_host *vha,
 			    entry->error_subcode2);
 			ha->tgt.tgt_ops->free_mcmd(mcmd);
 		}
-	} else {
+	} else if (mcmd) {
 		ha->tgt.tgt_ops->free_mcmd(mcmd);
 	}
 }