Fix use of variable after pfree

Hi,

While going through the code of the slot sync worker, I found that in
functions ReplSlotSyncWorkerMain and pg_sync_replication_slots the
variable app_name.data is being used after it is freed.

We can get logs as following:
2025-09-02 12:26:48.520 IST [3908359] ERROR: synchronization worker
"" could not connect to the primary server: connection to server at
"localhost" (127.0.0.1), port 5432 failed: Connection refused
Is the server running on that host and accepting TCP/IP connections?

I have moved the pfree(app_data.name) after its usage.

This change was introduced in PG_18.
The patch applies in the HEAD and REL_18_STABLE branches.

Thanks,
Shlok Kyal