main: Fix warning when client disappears

When a client disappears, check whether there is a driver to disable
before turning off polling.

Fixes this warning:
CRITICAL **: driver_set_polling: assertion 'driver' failed

src/iio-sensor-proxy.c

@@ -299,7 +299,8 @@ client_release (SensorData            *data,
 
 	g_hash_table_remove (ht, sender);
 
-	if (g_hash_table_size (ht) == 0)
+	if (driver_type_exists (data, driver_type) &&
+	    g_hash_table_size (ht) == 0)
 		driver_set_polling (DRIVER_FOR_TYPE(driver_type), FALSE);
 }