Sanity check for when the end tag is on the last line instead of a ne…

…w line

For example:
/*
 *
 * */

CodeSniffer/Tokenizers/Comment.php

@@ -200,6 +200,10 @@ private function _processLine($string, $eolChar, $start, $end)
             $start   += strlen($space['content']);
         }
 
+        if (isset($string[$start]) === false) {
+            return $tokens;
+        }
+
         if ($string[$start] === '@') {
             // The content up until the first whitespace is the tag name.
             $tagName = '';