Minor - fix PHP warning.

main/install/install.lib.php

@@ -1316,11 +1316,11 @@ function display_database_settings_form(
 ) {
     if ($installType == 'update') {
         global $_configuration;
-        $dbHostForm         = $_configuration['db_host'];
-        $dbUsernameForm     = $_configuration['db_user'];
-        $dbPassForm         = $_configuration['db_password'];
-        $dbNameForm         = $_configuration['main_database'];
-        $dbPortForm         = $_configuration['db_port'];
+        $dbHostForm = $_configuration['db_host'];
+        $dbUsernameForm = $_configuration['db_user'];
+        $dbPassForm = $_configuration['db_password'];
+        $dbNameForm = $_configuration['main_database'];
+        $dbPortForm = isset($_configuration['db_port']) ? $_configuration['db_port'] : '';
 
         echo '<div class="RequirementHeading"><h2>' . display_step_sequence() .get_lang('DBSetting') . '</h2></div>';
         echo '<div class="RequirementContent">';