work with non existing apps directory. thanks toAntonio for the hint

lib/installer.php

@@ -287,7 +287,7 @@ class OC_Installer{
 	 * This function installs all apps found in the 'apps' directory that should be enabled by default;
 	 */
 	public static function installShippedApps(){
-		$dir = opendir( OC::$APPSROOT."/apps" );
+		if($dir = opendir( OC::$APPSROOT."/apps" )){
 			while( false !== ( $filename = readdir( $dir ))){
 				if( substr( $filename, 0, 1 ) != '.' and is_dir(OC::$APPSROOT."/apps/$filename") ){
 					if( file_exists( OC::$APPSROOT."/apps/$filename/appinfo/app.php" )){
@@ -304,6 +304,7 @@ class OC_Installer{
 			}
 			closedir( $dir );
 		}
+	}
 
 	/**
 	 * install an app already placed in the app folder