dbus-marshal-validate: Check brackets in signature nest correctly

In debug builds with assertions enabled, a signature with incorrectly
nested `()` and `{}`, for example `a{i(u}` or `(a{ii)}`, could result
in an assertion failure.

In production builds without assertions enabled, a signature with
incorrectly nested `()` and `{}` could potentially result in a crash
or incorrect message parsing, although we do not have a concrete example
of either of these failure modes.

Thanks: Evgeny Vereshchagin
Resolves: https://gitlab.freedesktop.org/dbus/dbus/-/issues/418
Resolves: CVE-2022

-42010
Signed-off-by: Simon McVittie <smcv@collabora.com>