Description:
------------
When imap_mail_compose() is used to compose a multipart message
with one or more message/rfc822 parts where the subtype is
explicitly specified, the function segfaults.

Note that composing a multipart message with one or more
message/rfc822 where the subtype is not explicitly specified does
not segfault, due to the way this is handled by lib-client (2007).


Test script:
---------------
<?php
$bodies = [[
    'type' => TYPEMULTIPART,
], [
    'type' => TYPETEXT,
    'contents.data' => 'some text',
], [
    'type' => TYPEMESSAGE,
    'subtype' => 'RFC822',
]];
imap_mail_compose([], $bodies);
echo "done\n";
?>


Expected result:
----------------
done


Actual result:
--------------
segfault