Fixed Kanji encoding in some specific cases

The mb_detect_encoding can return false and we have to handle this
situation.

Fixes #11786

Signed-off-by: Michal Čihař <michal@cihar.com>

ChangeLog

@@ -8,6 +8,7 @@ phpMyAdmin - ChangeLog
 - issue #11783 Avoid syntax error in javascript messages on invalid PHP setting for max_input_vars
 - issue #11784 Properly handle errors in upacking zip archive
 - issue #11785 Set PHP's internal encoding to UTF-8
+- issue #11786 Fixed Kanji encoding in some specific cases
 
 4.5.3.1 (2015-12-25)
 - issue #11774 Undefined offset 2

libraries/kanji-encoding.lib.php

@@ -82,6 +82,9 @@ function PMA_Kanji_strConv($str, $enc, $kana)
         return $str;
     }
     $string_encoding = mb_detect_encoding($str, $kanji_encoding_list);
+    if ($string_encoding === false) {
+        $string_encoding = 'utf-8';
+    }
 
     if ($kana == 'kana') {
         $dist = mb_convert_kana($str, 'KV', $string_encoding);