[llvmpipe] Ensure valid scene pointer

Change-Id: I51d54084af8b6208d1d402c3cc5538f8cf7c7941
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1386459
Commit-Queue: Craig Stout <cstout@google.com>
Reviewed-by: David Gilhooley <dgilhooley@google.com>