Gracefully handle early fatal errors in AJAX requests

Generate response manually when Response object is not yet ready to be
used but we need AJAX response.

Fixes #13092

Signed-off-by: Michal Čihař <michal@cihar.com>

ChangeLog

@@ -11,6 +11,7 @@ phpMyAdmin - ChangeLog
 - issue #13362 Fixed loading additional javascripts
 - issue #13343 Fixed editing QBE
 - issue #13193 Improved documentation on user settings
+- issue #13092 Gracefully handle early fatal errors in AJAX requests
 
 4.7.1 (2017-05-25)
 - issue #13132 Always execute tracking queries as controluser

libraries/core.lib.php

@@ -222,17 +222,18 @@ function PMA_fatalError($error_message, $message_args = null) {
     }
 
     /*
-     * Avoid using Response if Config is not yet loaded
+     * Avoid using Response class as config does not have to be loaded yet
      * (this can happen on early fatal error)
      */
-    if (isset($GLOBALS['Config'])) {
-        $response = Response::getInstance();
-    } else {
-        $response = null;
-    }
-    if (! is_null($response) && $response->isAjax()) {
-        $response->setRequestStatus(false);
-        $response->addJSON('message', PMA\libraries\Message::error($error_message));
+    if (! empty($_REQUEST['ajax_request'])) {
+        // Generate JSON manually
+        PMA_headerJSON();
+        echo json_encode(
+            array(
+                'success' => false,
+                'error' => Message::error($error_message)->getDisplay(),
+            )
+        );
     } else {
         $error_message = strtr($error_message, array('<br />' => '[br]'));