Avoid direct execution of installer includes (#1488895)

installer/check.php

@@ -1,3 +1,10 @@
+<?php
+
+if (!class_exists('rcube_install') || !is_object($RCI)) {
+    die("Not allowed! Please open installer/index.php instead.");
+}
+
+?>
 <form action="index.php" method="get">
 <?php
 

installer/config.php

@@ -1,3 +1,10 @@
+<?php
+
+if (!class_exists('rcube_install') || !is_object($RCI)) {
+    die("Not allowed! Please open installer/index.php instead.");
+}
+
+?>
 <form action="index.php" method="post">
 <input type="hidden" name="_step" value="2" />
 <?php

installer/test.php

@@ -1,3 +1,10 @@
+<?php
+
+if (!class_exists('rcube_install') || !is_object($RCI)) {
+    die("Not allowed! Please open installer/index.php instead.");
+}
+
+?>
 <form action="index.php?_step=3" method="post">
 
 <h3>Check config files</h3>