Database: Ensure that variables are defined correctly.

There were some cases where some local variables in `wpdb::init_charset()` could be undefined when they were used.

Props Frozzare.
Fixes #37683 for trunk.



git-svn-id: https://develop.svn.wordpress.org/trunk@38439 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/wp-db.php

@@ -734,6 +734,9 @@ class wpdb {
 	 * @since 3.1.0
 	 */
 	public function init_charset() {
+		$charset = '';
+		$collate = '';
+
 		if ( function_exists('is_multisite') && is_multisite() ) {
 			$charset = 'utf8';
 			if ( defined( 'DB_COLLATE' ) && DB_COLLATE ) {