Replace 'thunar_return_if_fail (THUNAR_IS_DEVICE (device))' with

standard 'if (..)' to prevent possible crashes. (Bug #13404)

('_thunar_return_if_fail' will only be executed when compiled with
debugging enabled. )

'THUNAR_IS_DEVICE()' will be used over 'device != NULL', since that call
does a bit more than checking for NULL.

Co-authored-by: Alexander Schwinn <alexxcons@xfce.org>