pan/bi: Assert l != NULL in bi_ra

Confuses cppcheck; indeed, the proof is confusing.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12530>

src/panfrost/bifrost/bi_ra.c

@@ -535,6 +535,7 @@ bi_register_allocate(bi_context *ctx)
         }
 
         assert(success);
+        assert(l != NULL);
 
         ctx->info->tls_size = spill_count;
         bi_install_registers(ctx, l);