Issue #1215104 follow-up by chx: Fix unset of global variable.

core/includes/install.core.inc

@@ -101,7 +101,7 @@ function install_drupal($settings = array()) {
   // @see drupal_installation_attempted()
   $state = $install_state;
   if (!empty($install_state['installation_finished'])) {
-    unset($install_state);
+    unset($GLOBALS['install_state']);
   }
 
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