MDL-77735 core: Check $CFG->lang isset

In some cases, $CFG->lang might not be set, and this is causing a
Notice to be displayed when, for instance, database connection fails.
This patch should fix this case.

lib/weblib.php

@@ -2243,7 +2243,7 @@ function get_html_lang($dir = false) {
     global $CFG;
 
     $currentlang = current_language();
-    if ($currentlang !== $CFG->lang && !get_string_manager()->translation_exists($currentlang)) {
+    if (isset($CFG->lang) && $currentlang !== $CFG->lang && !get_string_manager()->translation_exists($currentlang)) {
         // Use the default site language when the current language is not available.
         $currentlang = $CFG->lang;
         // Fix the current language.