This crash happened with certain bad shaders which used temporary
values as array (or other) index expresisons.

Fixes the crash covered in the WebGL test
"conformance/bugs/undefined-index-should-not-crash"

BUG=angle:857

Change-Id: I13e2ba6d5f1ab0846ac902021bc0b57cbb37d759
Reviewed-on: https://chromium-review.googlesource.com/237460Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>