dblink: Fix variable confusion introduced in e4602483e9

Thanks to Robins to find the issue and Nathan for promptly writing a test case
to prevent future problems like this.

Reported-by: Nathan Bossart <nathandbossart@gmail.com>
Reported-by: Robins Tharakan <tharakan@gmail.com>
Author: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/20230130193008.GA2740781@nathanxps13

contrib/dblink/dblink.c

@@ -321,7 +321,7 @@ dblink_connect(PG_FUNCTION_ARGS)
 	else
 	{
 		if (pconn->conn)
-			libpqsrv_disconnect(conn);
+			libpqsrv_disconnect(pconn->conn);
 		pconn->conn = conn;
 	}
 

contrib/dblink/expected/dblink.out

@@ -938,6 +938,25 @@ DROP SERVER fdtest;
 ALTER FOREIGN DATA WRAPPER dblink_fdw OPTIONS (nonexistent 'fdw');
 ERROR:  invalid option "nonexistent"
 HINT:  There are no valid options in this context.
+-- test repeated calls to dblink_connect
+SELECT dblink_connect(connection_parameters());
+ dblink_connect 
+----------------
+ OK
+(1 row)
+
+SELECT dblink_connect(connection_parameters());
+ dblink_connect 
+----------------
+ OK
+(1 row)
+
+SELECT dblink_disconnect();
+ dblink_disconnect 
+-------------------
+ OK
+(1 row)
+
 -- test asynchronous notifications
 SELECT dblink_connect(connection_parameters());
  dblink_connect 

contrib/dblink/sql/dblink.sql

@@ -489,6 +489,11 @@ DROP SERVER fdtest;
 -- should fail
 ALTER FOREIGN DATA WRAPPER dblink_fdw OPTIONS (nonexistent 'fdw');
 
+-- test repeated calls to dblink_connect
+SELECT dblink_connect(connection_parameters());
+SELECT dblink_connect(connection_parameters());
+SELECT dblink_disconnect();
+
 -- test asynchronous notifications
 SELECT dblink_connect(connection_parameters());