Caused a panic when input was not valid. This is only relevant to the test suite
and doesn't occur anywhere else (apart from the previous similar commit for
lib-sieve-tool).

Panic was:

Panic in file smtp-address.c: line 684 (smtp_address_write): assertion failed: (smtp_char_is_qpair(*p))`

It is immediately referenced in sieve_interpreter_continue(), called from there.

The error string is currently always used and it must be, so there is no need to
allow for the error_r paramerter to be NULL and unused.

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.