Fix use-after-free issue in slot synchronization.

Author: Shlok Kyal <shlok.kyal.oss@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 18, where it was introduced
Discussion: https://postgr.es/m/CANhcyEXMrcEdzj-RNGJam0nJHM4y+ttdWsgUCFmXciM7BNKc7A@mail.gmail.com

src/backend/replication/logical/slotsync.c

@@ -1477,7 +1477,6 @@ ReplSlotSyncWorkerMain(const void *startup_data, size_t startup_data_len)
 	 */
 	wrconn = walrcv_connect(PrimaryConnInfo, false, false, false,
 							app_name.data, &err);
-	pfree(app_name.data);
 
 	if (!wrconn)
 		ereport(ERROR,
@@ -1485,6 +1484,8 @@ ReplSlotSyncWorkerMain(const void *startup_data, size_t startup_data_len)
 				errmsg("synchronization worker \"%s\" could not connect to the primary server: %s",
 					   app_name.data, err));
 
+	pfree(app_name.data);
+
 	/*
 	 * Register the disconnection callback.
 	 *

src/backend/replication/slotfuncs.c

@@ -921,7 +921,6 @@ pg_sync_replication_slots(PG_FUNCTION_ARGS)
 	/* Connect to the primary server. */
 	wrconn = walrcv_connect(PrimaryConnInfo, false, false, false,
 							app_name.data, &err);
-	pfree(app_name.data);
 
 	if (!wrconn)
 		ereport(ERROR,
@@ -929,6 +928,8 @@ pg_sync_replication_slots(PG_FUNCTION_ARGS)
 				errmsg("synchronization worker \"%s\" could not connect to the primary server: %s",
 					   app_name.data, err));
 
+	pfree(app_name.data);
+
 	SyncReplicationSlots(wrconn);
 
 	walrcv_disconnect(wrconn);