core: avoid unexporting active-connection that is not exported

For better or worse, nm_exported_object_unexport() asserts that the
object is currently exported.

It's not clear that an active connection at this place is always
exported.

Fixes: f0e3dfdace
(cherry picked from commit c8a649b3de)

src/nm-active-connection.c

@@ -167,6 +167,7 @@ _settings_connection_removed (NMSettingsConnection *connection,
 	 * re-link; in that case we'd just clean the references to the old connection here).
 	 * Let's remove ourselves from the bus so that we're not exposed with a dangling
 	 * reference to the setting connection once it's gone. */
+	if (nm_exported_object_is_exported (NM_EXPORTED_OBJECT (self)))
 		nm_exported_object_unexport (NM_EXPORTED_OBJECT (self));
 }