WPDB: Check that `AUTH_SALT` is not empty.

In `wpdb::placeholder_escape()`, the key for `hash_hmac()` defaults to `AUTH_SALT`, but `hash_hmac()` will return an empty string if the key is empty.

This had the side effect of the string `{}` being incorrectly replaced with a `%` character in queries just about to be run on the database.

Props jsonfry.
Fixes #42431.


Built from https://develop.svn.wordpress.org/trunk@42120


git-svn-id: http://core.svn.wordpress.org/trunk@41949 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-RC1-42119';
+$wp_version = '4.9-RC1-42120';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.

wp-includes/wp-db.php

@@ -1946,7 +1946,7 @@ class wpdb {
 			// If ext/hash is not present, compat.php's hash_hmac() does not support sha256.
 			$algo = function_exists( 'hash' ) ? 'sha256' : 'sha1';
 			// Old WP installs may not have AUTH_SALT defined.
-			$salt = defined( 'AUTH_SALT' ) ? AUTH_SALT : (string) rand();
+			$salt = defined( 'AUTH_SALT' ) && AUTH_SALT ? AUTH_SALT : (string) rand();
 
 			$placeholder = '{' . hash_hmac( $algo, uniqid( $salt, true ), $salt ) . '}';
 		}