Fix #80216: imap_mail_compose() does not validate types/encodings

We need to check whether the given `type`s and `encoding`s are within
bounds to avoid segfaults and out-of-bound reads.

Fix #80216: imap_mail_compose() does not validate types/encodings

We need to check whether the given `type`s and `encoding`s are within
bounds to avoid segfaults and out-of-bound reads.

Closes GH-6323.

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