device: add null check on `self->priv->modem' in clear_modem

clear_modem() can be invoked from set_property() and dispose(), where
`self->priv->modem' may be NULL. This patch adds a null check on the
modem object to make sure we don't run g_object_run_dispose() on a null
modem object.

src/mm-device.c

@@ -302,10 +302,12 @@ clear_modem (MMDevice *self)
         self->priv->modem_valid_id = 0;
     }
 
+    if (self->priv->modem) {
         /* Run dispose before unref-ing, in order to cleanup the SIM object,
          * if any (which also holds a reference to the modem object) */
         g_object_run_dispose (G_OBJECT (self->priv->modem));
         g_clear_object (&(self->priv->modem));
+    }
 }
 
 void