portal: Check if the monitor is closed before emitting progress

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`emit_progress` is called from a thread while another thread could
unexport/close the GDbusInterfaceSkeleton. If that happens, the
connection becomes NULL which is illegal for
`g_dbus_connection_emit_signal` which results in dereferencing an unset
error.

The fix is to lock the UpdateMonitorData, and then only emit the signal
if it is not closed.

(cherry picked from commit ce4a27e1fe):

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