Require numeric IDs in user deletion functions.

`wp_delete_user()` and `wpmu_delete_user()` both require an `$id` parameter.
Previously, the functions did not verify that the value passed was, in fact,
a number. As such, passing an object or any other entity that would be cast
to int `1` would result in user 1 being deleted. We fix this by enforcing
the requirement that `$id` be numeric.

Props dipesh.kakadiya, utkarshpatel, juliobox.
Fixes #33800.

git-svn-id: https://develop.svn.wordpress.org/trunk@34034 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/user.php

@@ -273,6 +273,10 @@ function get_users_drafts( $user_id ) {
 function wp_delete_user( $id, $reassign = null ) {
 	global $wpdb;
 
+	if ( ! is_numeric( $id ) ) {
+		return false;
+	}
+
 	$id = (int) $id;
 	$user = new WP_User( $id );