AlterSubscription_refresh: avoid stomping on global variable

This patch replaces use of the global "wrconn" variable in
AlterSubscription_refresh with a local variable of the same name, making
it consistent with other functions in subscriptioncmds.c (e.g.
DropSubscription).

The global wrconn is only meant to be used for logical apply/tablesync worker.
Abusing it this way is known to cause trouble if an apply worker
manages to do a subscription refresh, such as reported by Jeremy Finzel
and diagnosed by Andres Freund back in November 2020, at
https://www.postgresql.org/message-id/20201111215820.qihhrz7fayu6myfi@alap3.anarazel.de

Backpatch to 10.  In branch master, also move the connection establishment
to occur outside the PG_TRY block; this way we can remove a test for NULL in
PG_FINALLY, and it also makes the code more consistent with similar code in
the same file.

Author: Peter Smith <peter.b.smith@fujitsu.com>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: Japin Li <japinli@hotmail.com>
Discussion: https://postgr.es/m/CAHut+Pu7Jv9L2BOEx_Z0UtJxfDevQSAUW2mJqWU+CtmDrEZVAg@mail.gmail.com

src/backend/commands/subscriptioncmds.c

@@ -556,18 +556,19 @@ AlterSubscription_refresh(Subscription *sub, bool copy_data)
 		char		state;
 	} SubRemoveRels;
 	SubRemoveRels *sub_remove_rels;
+	WalReceiverConn *wrconn;
 
 	/* Load the library providing us libpq calls. */
 	load_file("libpqwalreceiver", false);
 
-	PG_TRY();
-	{
 	/* Try to connect to the publisher. */
 	wrconn = walrcv_connect(sub->conninfo, true, sub->name, &err);
 	if (!wrconn)
 		ereport(ERROR,
 				(errmsg("could not connect to the publisher: %s", err)));
 
+	PG_TRY();
+	{
 		/* Get the table list from publisher. */
 		pubrel_names = fetch_table_list(wrconn, sub->publications);
 
@@ -737,7 +738,6 @@ AlterSubscription_refresh(Subscription *sub, bool copy_data)
 	}
 	PG_FINALLY();
 	{
-		if (wrconn)
 		walrcv_disconnect(wrconn);
 	}
 	PG_END_TRY();
@@ -1062,7 +1062,7 @@ DropSubscription(DropSubscriptionStmt *stmt, bool isTopLevel)
 	ListCell   *lc;
 	char		originname[NAMEDATALEN];
 	char	   *err = NULL;
-	WalReceiverConn *wrconn = NULL;
+	WalReceiverConn *wrconn;
 	Form_pg_subscription form;
 	List	   *rstates;