Fix "Illegal string offset" warning in rcube::log_bug() on PHP 7.1 (#5508)

CHANGELOG

@@ -12,6 +12,7 @@ CHANGELOG Roundcube Webmail
 - Fix bug where IMAP password could be exposed via error message (#5472)
 - Fix bug where it wasn't possible to store more that 2MB objects in memcache/apc,
   Added memcache_max_allowed_packet and apc_max_allowed_packet settings (#5452)
+- Fix "Illegal string offset" warning in rcube::log_bug() on PHP 7.1 (#5508)
 
 RELEASE 1.2.2
 -------------

program/lib/Roundcube/rcube.php

@@ -1322,7 +1322,6 @@ class rcube
 
         // write error to local log file
         if (($level & 1) || !empty($arg_arr['fatal'])) {
-            $post_query = '';
             if ($_SERVER['REQUEST_METHOD'] == 'POST') {
                 foreach (array('_task', '_action') as $arg) {
                     if ($_POST[$arg] && !$_GET[$arg]) {