Bug#1735072: Fix upgrade error by delimiting event_log table in sql

Forgot to wrap the table name event_log with curly brackets.

Sponsored by Australian National University

behatnotneeded

Change-Id: Ia598632b1f0ae177fe646312ea5c2b38a9e83d51

htdocs/lib/db/upgrade.php

@@ -5127,7 +5127,7 @@ function xmldb_core_upgrade($oldversion=0) {
             $field->setAttributes(XMLDB_TYPE_DATETIME, null, null);
             add_field($table, $field);
 
-            $sql = "update event_log set ctime = time";
+            $sql = "UPDATE {event_log} SET ctime = time";
             execute_sql($sql);
 
             // now set ctime attribute to NOT NULL.