Fix regression in dblink_disconnect() reported by Eduardo Stern:

persistent_conn was left dangling after a disconnect in the
unnamed connection case, causing a subsequent disconnect to
crash the backend.

contrib/dblink/dblink.c

@@ -256,6 +256,8 @@ dblink_disconnect(PG_FUNCTION_ARGS)
 		deleteConnection(conname);
 		pfree(rcon);
 	}
+	else
+		persistent_conn = NULL;
 
 	PG_RETURN_TEXT_P(GET_TEXT("OK"));
 }

contrib/dblink/expected/dblink.out

@@ -185,13 +185,11 @@ SELECT dblink_disconnect();
  OK
 (1 row)
 
--- should generate "no connection to the server" error
+-- should generate "connection not available" error
 SELECT *
 FROM dblink('SELECT * FROM foo') AS t(a int, b text, c text[])
 WHERE t.a > 7;
-ERROR:  sql error
+ERROR:  connection not available
-DETAIL:  no connection to the server
-
 -- put more data into our slave table, first using arbitrary connection syntax
 -- but truncate the actual return value so we can use diff to check for success
 SELECT substr(dblink_exec('dbname=regression','INSERT INTO foo VALUES(10,''k'',''{"a10","b10","c10"}'')'),1,6);

contrib/dblink/sql/dblink.sql

@@ -105,7 +105,7 @@ FROM dblink_fetch('rmt_foo_cursor',4) AS t(a int, b text, c text[]);
 -- close the persistent connection
 SELECT dblink_disconnect();
 
--- should generate "no connection to the server" error
+-- should generate "connection not available" error
 SELECT *
 FROM dblink('SELECT * FROM foo') AS t(a int, b text, c text[])
 WHERE t.a > 7;