panfrost: Assert staging resource allocation was successful

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10266>

src/gallium/drivers/panfrost/pan_resource.c

@@ -824,6 +824,7 @@ panfrost_ptr_map(struct pipe_context *pctx,
         /* We don't have s/w routines for AFBC, so use a staging texture */
         if (drm_is_afbc(rsrc->image.layout.modifier)) {
                 struct panfrost_resource *staging = pan_alloc_staging(ctx, rsrc, level, box);
+                assert(staging);
 
                 /* Staging resources have one LOD: level 0. Query the strides
                  * on this LOD.