DBusMessageIter: Zero out the iterator on failure

DBusMessageIter: Zero out the iterator on failure

This ensures that callers won't accidentally use it for something
in a way that is considered to be programmer error.

In _dbus_message_iter_check(), insert a specific check for this before
dereferencing iter->message, so that we get a nice assertion failure
(potentially non-fatal) instead of a segfault.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=101568