This prevents nullptr dereferences when invalid OpConstantComposite
instructions make it past spirv-val.
Fixed: chromium:1231650
Change-Id: Iddfe04dcfdce382096ae7dec7af8718b579f7951
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/61080
Auto-Submit: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: David Neto <dneto@google.com>