r300g: fix crash when destroying the context on r3xx-r4xx

FDO bug #28652,

src/gallium/drivers/r300/r300_context.c

@@ -45,7 +45,7 @@ static void r300_destroy_context(struct pipe_context* context)
 
     if (r300->texkill_sampler) {
         pipe_sampler_view_reference(
-                (struct pipe_sampler_view**)r300->texkill_sampler,
+                (struct pipe_sampler_view**)&r300->texkill_sampler,
                 NULL);
     }