Fix potential null pointer dereference.

Change-Id: I12249cc2e647296eb45f9e7f9082fe3b477b9011
Reviewed-on: https://swiftshader-review.googlesource.com/3463
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>