check return on both iconv calls

git-svn-id: http://svn.php.net/repository/pear/packages/Mail_Mime/trunk@317689 c90b9560-bf6c-de11-be94-00142212c4b1

mimeDecode.php

@@ -758,7 +758,8 @@ function _decodeHeader($input, $default_charset=false)
                     break;
             }
             if (is_string($this->_decode_headers)) {
-                $text = @iconv($charset, $this->_decode_headers, $text);
+                $conv = @iconv($charset, $this->_decode_headers, $text);
+                $text = ($conv === false) ? $text : $conv;
             }
             $input = str_replace($encoded, $text, $input);
         }