st/nine: Fix volumetexture dtor on ctor failure

The dtor is called on allocation failure,
thus we must check the volumes are allocated
before trying to release them.

Signed-off-by: Axel Davy <davyaxel0@gmail.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit f91f748f)