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

Author: alvherre

src/backend/commands/subscriptioncmds.c

@@ -517,6 +517,7 @@ AlterSubscription_refresh(Subscription *sub, bool copy_data)
 	List	   *subrel_states;
 	Oid		   *subrel_local_oids;
 	Oid		   *pubrel_local_oids;
+	WalReceiverConn *wrconn;
 	ListCell   *lc;
 	int			off;
 
@@ -835,7 +836,7 @@ DropSubscription(DropSubscriptionStmt *stmt, bool isTopLevel)
 	char		originname[NAMEDATALEN];
 	char	   *err = NULL;
 	RepOriginId originid;
-	WalReceiverConn *wrconn = NULL;
+	WalReceiverConn *wrconn;
 	StringInfoData cmd;
 	Form_pg_subscription form;