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

[cmb69]

We need to check whether the given types and encodings are within
bounds to avoid segfaults and out-of-bound reads.

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This is ugly. In order to detect whether the custom types and encodings are actually supported, we have to access the global body_types and body_encodings which are declared in mail.h, but not be exported. There is no issue on Windows, because libc-client is linked statically (at least with the official dependencies), but otherwise this patch may not even build.

[nikic]

This is ugly. In order to detect whether the custom types and encodings are actually supported, we have to access the global body_types and body_encodings which are declared in mail.h, but not be exported. There is no issue on Windows, because libc-client is linked statically (at least with the official dependencies), but otherwise this patch may not even build.

Presumably libc-client compiles using default visibility rather than -fvisibility=hidden, so I believe those symbols should also be visible in shared libraries.

[cmb69]

Thanks! Then we should go with this.