Themes: Avoid PHP warnings in the admin when no theme directory exists.

Props iseulde, chris_dev, swissspidy.
Fixes #35521.

Built from https://develop.svn.wordpress.org/trunk@40636


git-svn-id: http://core.svn.wordpress.org/trunk@40497 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/theme-install.php

@@ -28,6 +28,11 @@ if ( ! is_network_admin() ) {
 }
 
 $installed_themes = search_theme_directories();
+
+if ( false === $installed_themes ) {
+	$installed_themes = array();
+}
+
 foreach ( $installed_themes as $k => $v ) {
 	if ( false !== strpos( $k, '/' ) ) {
 		unset( $installed_themes[ $k ] );

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.8-alpha-40635';
+$wp_version = '4.8-alpha-40636';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.