Fix pgsql use after free trying to reuse closed connection

When a connection is closed, we also need to remove the hash entry
from the regular_list, as it now points to freed memory. To do this
store a reverse mapping from the connection to the hash string.

It would be nicer to introduce a wrapping structure for the pgsql
link resource that could store the hash (and notices), but that would
require large changes to the extension, so I'm going for a more
minimal fix here.

ext/pgsql/tests/connect_after_close.phpt

@@ -0,0 +1,19 @@
+--TEST--
+Reopen connection after it was closed
+--SKIPIF--
+<?php include("skipif.inc"); ?>
+--FILE--
+<?php
+include('config.inc');
+
+/* Run me under valgrind */
+$db1 = pg_connect($conn_str);
+unset($db1);
+var_dump(pg_close());
+$db2 = pg_connect($conn_str);
+unset($db2);
+var_dump(pg_close());
+?>
+--EXPECT--
+bool(true)
+bool(true)