iscsi-scst: Suppress a Coverity taint complaint

Suppress the following (false positive) Coverity complaint:

    CID 271578 (#1 of 1): Dereference after null check (FORWARD_NULL)
    var_deref_model: Passing null pointer (*ref_cmd).scst_cmd to
    scst_set_delivery_status, which dereferences it

(*ref_cmd).scst_aen is set when (*ref_cmd).scst_state == ISCSI_CMD_STATE_AEN
and vice versa, so the Coverity complaint is a false positive. Hence rewrite
the code to suppress this complaint and make the code cleaner.

iscsi-scst/kernel/nthread.c

@@ -1315,15 +1315,19 @@ static int write_data(struct iscsi_conn *conn)
 			    (unsigned long long)conn->session->sid,
 			    conn->cid, conn->write_cmnd);
 	}
-	if (ref_cmd_to_parent &&
-	    ((ref_cmd->scst_cmd != NULL) || (ref_cmd->scst_aen != NULL))) {
-		if (ref_cmd->scst_state == ISCSI_CMD_STATE_AEN)
-			scst_set_aen_delivery_status(ref_cmd->scst_aen,
-				SCST_AEN_RES_FAILED);
-		else
-			scst_set_delivery_status(ref_cmd->scst_cmd,
-				SCST_CMD_DELIVERY_FAILED);
+
+	if (ref_cmd_to_parent) {
+		if (ref_cmd->scst_state == ISCSI_CMD_STATE_AEN) {
+			if (ref_cmd->scst_aen)
+				scst_set_aen_delivery_status(ref_cmd->scst_aen,
+					SCST_AEN_RES_FAILED);
+		} else {
+			if (ref_cmd->scst_cmd)
+				scst_set_delivery_status(ref_cmd->scst_cmd,
+					SCST_CMD_DELIVERY_FAILED);
+		}
 	}
+
 	goto out;
 }