Only update the old allowed_themes network option from the main site admin or in the network admin. see #20103, #20146.

git-svn-id: http://svn.automattic.com/wordpress/trunk@20145 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-theme.php

@@ -1103,7 +1103,7 @@ final class WP_Theme implements ArrayAccess {
 				$allowed_themes = $converted;
 			}
 			// Set the option so we never have to go through this pain again.
-			if ( is_admin() ) {
+			if ( ( is_admin() && is_main_site() ) || is_network_admin() ) {
 				update_site_option( 'allowedthemes', $allowed_themes );
 				delete_site_option( 'allowed_themes' );
 			}