Code Quality: Initialize $message and $message_class in network_step1().

This prevents the variables from being undefined when the condition isn't met.

Follow-up to [56599].

Props huzaifaalmesbah, noruzzaman.
See #64238.

git-svn-id: https://develop.svn.wordpress.org/trunk@61753 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/network.php

@@ -198,6 +198,9 @@ function network_step1( $errors = false ) {
 	} else {
 		$subdomain_install = false;
 		$got_mod_rewrite   = got_mod_rewrite();
+		$message_class     = '';
+		$message           = '';
+
 		if ( $got_mod_rewrite ) { // Dangerous assumptions.
 			$message_class = 'updated';
 			$message       = '<p><strong>' . __( 'Warning:' ) . '</strong> ';