Multisite: Return `get_current_blog_id()` value instead of hard-coded 1 in `get_main_site_id()` for non-multisite environments.

See #41936.

Built from https://develop.svn.wordpress.org/trunk@41862


git-svn-id: http://core.svn.wordpress.org/trunk@41696 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/functions.php

@@ -4423,7 +4423,7 @@ function is_main_site( $site_id = null, $network_id = null ) {
  */
 function get_main_site_id( $network_id = null ) {
 	if ( ! is_multisite() ) {
-		return 1;
+		return get_current_blog_id();
 	}
 
 	$network = get_network( $network_id );

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-beta2-41861';
+$wp_version = '4.9-beta2-41862';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.