Fix unititialized string offset access in query parser

The code always tried to access positon 2 of the parsed token, even for
tokens shorter than that (like OR)

fixes #3896

_test/tests/inc/changelog_getrevisions.test.php

@@ -228,4 +228,5 @@ function test_requesttoolargenumberrevs() {
         $this->assertEquals($revsexpected, $revs);
     }
 
+
 }

inc/fulltext.php

@@ -845,7 +845,7 @@ function ft_queryParser($Indexer, $query){
     $q['parsed_ary'] = $parsed_ary;
 
     foreach ($q['parsed_ary'] as $token) {
-        if ($token[2] !== ':') continue;
+        if (strlen($token) < 3 || $token[2] !== ':') continue;
         $body = substr($token, 3);
 
         switch (substr($token, 0, 3)) {