DBusMessage: assert the properties we need DBusMessageIter to have

DBusMessage: assert the properties we need DBusMessageIter to have

We already asserted that DBusMessageIter must be at least as large
as DBusMessageRealIter (so that casting DBusMessageIter * to
DBusMessageRealIter * does not result in overflowing the stack
variable). Also assert that it must have alignment requirements at
least as strict as those of DBusMessageRealIter * (so that casting
does not increase the required alignment).

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Thiago Macieira <thiago@kde.org>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=94136