While debugging the 1.2.1 dEQP-VK.graphicsfuzz.* tests, noticed a crash
here where a non-constant failed to have its intermediate pointer set.

An assert here is clearer than exploding.

Bug: b/146653181
Change-Id: I23aae701edce1595cd145930d304c9f6e4b82924
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39808Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>