MDL-46212 questions: add {} whenver vars are put into "" strings

Having had one subtle bug cased be this, I decided to do a search and
fix all of these.

question/previewlib.php

@@ -259,7 +259,7 @@ function question_preview_question_pluginfile($course, $context, $component,
 
     $fs = get_file_storage();
     $relativepath = implode('/', $args);
-    $fullpath = "/$context->id/$component/$filearea/$relativepath";
+    $fullpath = "/{$context->id}/{$component}/{$filearea}/{$relativepath}";
     if (!$file = $fs->get_file_by_hash(sha1($fullpath)) or $file->is_directory()) {
         send_file_not_found();
     }