read after free in notmuch new

[David Bremner]

I haven't had a chance to really track this down, but it seems there is
a memory error in notmuch new (or a maybe false positive from valgrind).

Attached is the log from running "make memory-test OPTIONS=--medium" on
current git master (0e037c34).

It looks like we talloc the message_id string with the message object as
parent, but it somehow outlives the message object.

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