st/nine: Fix leak with cubetexture dtor

The last level was not released.

Signed-off-by: Axel Davy <axel.davy@ens.fr>
diff --git a/src/gallium/state_trackers/nine/cubetexture9.c b/src/gallium/state_trackers/nine/cubetexture9.c
index 0a834eb..977a345 100644
--- a/src/gallium/state_trackers/nine/cubetexture9.c
+++ b/src/gallium/state_trackers/nine/cubetexture9.c
@@ -173,7 +173,7 @@
     DBG("This=%p\n", This);
 
     if (This->surfaces) {
-        for (i = 0; i < This->base.base.info.last_level * 6; ++i)
+        for (i = 0; i < (This->base.base.info.last_level + 1) * 6; ++i)
             NineUnknown_Destroy(&This->surfaces[i]->base.base);
         FREE(This->surfaces);
     }