Properly fix #80220

The original fix for that bug[1] broke the formerly working composition
of message/rfc822 messages, which results in a segfault when freeing
the message body now.

Properly fix #80220

The original fix for that bug[1] broke the formerly working composition
of message/rfc822 messages, which results in a segfault when freeing
the message body now. While `imap_mail_compose()` does not really
support composition of meaningful message/rfc822 messages (although
libc-client appears to support that), some code may still use this to
compose partial messages, and using string manipulation to create the
final message.

The point is that libc-client expects `TYPEMESSAGE` with an explicit
subtype of `RFC822` to have a `nested.msg` (otherwise there will be a
segfault during free), but not to have any `contents.text.data` (this
will leak otherwise).

[1] <http://git.php.net/?p=php-src.git;a=commit;h=0d022ddf03c5fabaaa22e486d1e4a367ed9170a7>

Closes GH-6343.

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