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.



git-svn-id: https://develop.svn.wordpress.org/trunk@30299 602fd350-edb4-49c9-b593-d223f7449a82

src/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 );