The segfault happens when this match type is the last argument of the test
command. This situation is not possible in a valid script; positional arguments
are normally present after that, which would prevent the segfault. A variant of
this bug occurs when the match type also has no argument of its own. In either
case the segfault is caused by referring to absent tag arguments, which causes
a NULL dereference.

It's still not fully correct, but we don't have existing code that supports
parsing it.

Use mail_get_message_id() instead of just mail_get_first_header().

Previous commit was accidentally for an earlier version of the API change.

Found by latest clang scan-build.

Nothing changes in the code logic.

The old code seems to have worked correctly already, but now it's a bit
clearer how it was intended to work.

Fixes "warning: cast to smaller integer type" with new clang.