vrend: Add an assert for allocating the intermediate texture

With the format checks introduced earlier the double free bug reported
in #139 should no longer be possible. Nevertheless, add an assert to
ensure that this is checked.

Closes #139

v2: Update NDEBUG_UNUSED to MAYBE_UNUSED

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/vrend_renderer.c

@@ -8385,7 +8385,8 @@ static void vrend_renderer_blit_int(struct vrend_context *ctx,
       args.array_size = src_res->base.array_size;
       intermediate_copy = (struct vrend_resource *)CALLOC_STRUCT(vrend_texture);
       vrend_renderer_resource_copy_args(&args, intermediate_copy);
-      vrend_renderer_resource_allocate_texture(intermediate_copy, NULL);
+      MAYBE_UNUSED int r = vrend_renderer_resource_allocate_texture(intermediate_copy, NULL);
+      assert(!r);
 
       glGenFramebuffers(1, &intermediate_fbo);
    } else {