MDL-52983 excel output: avoid single quote on end of sheet name

This change resolves a problem where a long sheet name, which gets
trimmed down with substr($name, 0, 31), could result in a sheet name
with a single quote on the end because the quote removal was done before
the substr.

lib/excellib.class.php

@@ -163,6 +163,8 @@ public function __construct($name, PHPExcel $workbook) {
         $name = strtr(trim($name, "'"), '[]*/\?:', '       ');
         // Shorten the title if necessary.
         $name = core_text::substr($name, 0, 31);
+        // After the substr, we might now have a single quote on the end.
+        $name = trim($name, "'");
 
         if ($name === '') {
             // Name is required!