wpdb: When flushing results on a mysqli connection, make sure that `wpdb::$dbh` is a valid mysqli connection handle.

Fixes a unit test failure introduced in [30297].

Fixes #28155.

Props soulseekah.


Built from https://develop.svn.wordpress.org/trunk@30299


git-svn-id: http://core.svn.wordpress.org/trunk@30298 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.1-alpha-30298';
+$wp_version = '4.1-alpha-30299';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.

wp-includes/wp-db.php

@@ -1326,6 +1326,11 @@ class wpdb {
 			mysqli_free_result( $this->result );
 			$this->result = null;
 
+			// Sanity check before using the handle
+			if ( empty( $this->dbh ) || !( $this->dbh instanceof mysqli ) ) {
+				return;
+			}
+
 			// Clear out any results from a multi-query
 			while ( mysqli_more_results( $this->dbh ) ) {
 				mysqli_next_result( $this->dbh );