Memset the scalars to 0, and check their pointers before use. Catches cases where the intermediate was declared but not set.

Also switch from using assert() to ASSERT() as the latter still fires with DCHECK_ALWAYS_ON.

Change-Id: I81abb50aea41568f95c22a340b90b7c56839d3ce
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/25869Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>