mutt recipient parsing memory leak

[Tavis Ormandy <taviso () gmail com>]

Hello, I noticed mutt was leaking memory whenever I opened a particular
mailbox. I tracked down the problem: Using rfc822 groups without the madatory
labels wasn't being parsed properly.

https://tools.ietf.org/html/rfc822#section-6.2.6

(A spammer had just put some junk in there, they weren't deliberately using
exotic addressing schemes.. haha).

It turns out that you can send a small message that leaks a *lot* of memory. A
small message can leak GBs of memory, effectively preventing you from opening
your mailbox. You would need to use a different mail client to clean up the
malformed message before you can use mutt again.

I sent this upstream as a DoS, but they don't want to treat it as a security
isssue. I though I'd just send a FYI here instead in case anyone wants to
backport the patch.

Here's the bug with a repro: https://gitlab.com/muttmua/mutt/-/issues/323

Here's the patch:

https://gitlab.com/muttmua/mutt/-/commit/c059e20ea4c7cb3ee9ffd3500ffe313ae84b2545

Tavis.

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