Description:
------------
iconv_mime_encode(),'Q' encoding scheme isn't reliable and 
sometimes (for particular character and/or string length combination?) returns: "Notice: iconv_mime_encode(): Unknown error (7) in ..."
*without any result*. 

This also applies to earlier php versions (tested with php 5.2.1).



Reproduce code:
---------------
$preferences = array(
    "input-charset" => "UTF-8",
    "output-charset" => "UTF-8",
    "line-length" => 76,
    "line-break-chars" => "\n",
    "scheme" => "Q"
);

// $str1 results error, it's utf-8 string, its base64_encode() is: 
//'xIXEjcSZxJfEr8WhxbPFviDEr8SZxI3FocWzxJnEr8SFIMSNxJnFs8SFxaHFs8Wr'
$str1 = "ąčęėį?ų? įęč?ųęįą čęųą?ųū"; 

// $str2 doesn't result error, although it's only one character
// shorter. It's utf-8 string, its base64_encode() is: 
//'xIXEjcSZxJfEr8WhxbPFviDEr8SZxI3FocWzxJnEr8SFIMSNxJnFs8SFxaHFsw=='
$str2 = "ąčęėį?ų? įęč?ųęįą čęųą?ų";

echo iconv_mime_encode("Subject", $str1, $preferences);
echo iconv_mime_encode("Subject", $str2, $preferences);


Expected result:
----------------
Well, at least any (*some*) result is expected, without any 
errors and warnings. 

For $str1 is expected:
Subject: =?UTF-8?Q?=C4=85=C4=8D=C4=99=C4=97=C4=AF=C5=A1=C5=B3?=
 =?UTF-8?Q?=C5=BE=20=C4=AF=C4=99=C4=8D=C5=A1=C5=B3=C4=99=C4=AF?=
 =?UTF-8?Q?=C4=85=20=C4=8D=C4=99=C5=B3=C4=85=C5=A1=C5=B3=C5=AB?=

For $str2 is expected:
Subject: =?UTF-8?Q?=C4=85=C4=8D=C4=99=C4=97=C4=AF=C5=A1=C5=B3?=
 =?UTF-8?Q?=C5=BE=20=C4=AF=C4=99=C4=8D=C5=A1=C5=B3=C4=99=C4=AF?=
 =?UTF-8?Q?=C4=85=20=C4=8D=C4=99=C5=B3=C4=85=C5=A1=C5=B3?=

Actual result:
--------------
For $str1: 
FALSE with "Notice: iconv_mime_encode(): Unknown error (7) in ..."


For $str2:
Subject: =?UTF-8?Q?=C4=85=C4=8D=C4=99=C4=97=C4=AF=C5=A1=C5=B3?=
 =?UTF-8?Q?=C5=BE=20=C4=AF=C4=99=C4=8D=C5=A1=C5=B3=C4=99=C4=AF?=
 =?UTF-8?Q?=C4=85=20=C4=8D=C4=99=C5=B3=C4=85=C5=A1=C5=B3?=