Initialize dblink remoteConn struct in all cases

Two of the members of rconn were left uninitialized. When
dblink_open() is called without an outer transaction it
handles the initialization for us, but with an outer
transaction it does not. Arrange for initialization
in all cases. Backpatch to all supported versions.

Reported-by: Alexander Lakhin
Discussion: https://www.postgresql.org/message-id/flat/9bd0744f-5f04-c778-c5b3-809efe9c30c7%40joeconway.com#c545909a41664991aca60c4d70a10ce7

contrib/dblink/dblink.c

@@ -298,8 +298,13 @@ dblink_connect(PG_FUNCTION_ARGS)
 		conname_or_str = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
 	if (connname)
+	{
 		rconn = (remoteConn *) MemoryContextAlloc(TopMemoryContext,
 												  sizeof(remoteConn));
+		rconn->conn = NULL;
+		rconn->openCursorCount = 0;
+		rconn->newXactForCursor = false;
+	}
 
 	/* first check for valid foreign data server */
 	connstr = get_connect_string(conname_or_str);