6e581deb91 added an `spa_autoptr(DbusMessage) m`
for the new message sent out when a signal is received from modemmanager.
However this ended up shadowing the original `m` function arg,
so the code that wanted to interrogate the original arg with
`dbus_get_message_path` etc ended up interrogating this `NULL` value instead.
This triggered a NULL-check in `dbus_get_message_path` and caused
the process to abort.
Original downstream report: https://gitlab.com/postmarketOS/pmaports/-/issues/2886
2501b347ef