Fix leak when persistent PDO connection fails

As we don't register the resource, the resource dtor is not called
and will not decrement the refcount.

ext/pdo/pdo_dbh.c

@@ -419,6 +419,10 @@ options:
 	}
 
 	/* the connection failed; things will tidy up in free_storage */
+	if (is_persistent) {
+		dbh->refcount--;
+	}
+
 	/* XXX raise exception */
 	zend_restore_error_handling(&zeh);
 	if (!EG(exception)) {

ext/pdo_mysql/tests/construct_persistent_failure.phpt

@@ -0,0 +1,16 @@
+--TEST--
+Failure when creating persistent connection
+--EXTENSIONS--
+pdo_mysql
+--FILE--
+<?php
+try {
+    $pdo = new PDO('mysql:host=localhost', 'invalid_user', 'invalid_password', [
+        PDO::ATTR_PERSISTENT => true,
+    ]);
+} catch (PDOException $e) {
+    echo "Caught\n";
+}
+?>
+--EXPECT--
+Caught