sd-bus: add depth limit to message_skip_fields() to prevent stack overflow (#42164)

`message_skip_fields()` recursively processes D-Bus variant types in
message header fields with no depth limit. A crafted message with deeply
nested variants can cause unbounded recursion and overflow the stack.

Add a `depth` parameter checked against `BUS_CONTAINER_DEPTH` (128),
matching the limit already enforced by the public
`sd_bus_message_skip()` API. All recursive call sites pass `depth + 1`,
and the top-level caller in `message_parse_fields()` passes `0`.

diff --cc src/libsystemd/sd-bus/test-bus-marshal.c
Simple merge