Fix #80242: imap_mail_compose() segfaults for multipart with rfc822

libc-client expects `TYPEMESSAGE` with an explicit subtype of `RFC822`
to have a `nested.msg` (otherwise there will be

Fix #80242: imap_mail_compose() segfaults for multipart with rfc822

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).

Closes GH-6345.

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