[#11763] ysql: Correctly assign table descriptor in YbGetTableDescAnd…

…Props() when table doesn't exist

Summary:
In YbGetTableDescAndProps(), when allow_missing is true, the func would assign NULL to the table descriptor when table doesn't exist.

However, since desc is declared as `YBCPgTableDesc *`, assigning to desc itself wouldn't achieve the goal.

We should assign to `*desc` when the table is absent.

Test Plan:
For YB instance created without D15799:
```
bin/yb-admin -timeout_ms 180000 upgrade_ysql
YSQL successfully upgraded to the latest version
```
Run ysqlsh to connect to the YB instance

Reviewers: alex

Reviewed By: alex

Subscribers: yql

Differential Revision: https://phabricator.dev.yugabyte.com/D16009

src/postgres/src/backend/utils/misc/pg_yb_utils.c

@@ -1549,7 +1549,7 @@ YbGetTableDescAndProps(Oid table_oid,
 		HandleYBStatus(YBCPgTableExists(MyDatabaseId, table_oid, &exists_in_yb));
 		if (!exists_in_yb)
 		{
-			desc = NULL;
+			*desc = NULL;
 			return;
 		}
 	}