Fix a translator crash with index expressions. 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/237460 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>