TIntermTyped::CreateZero can be reached with a void array type in an
error case. Handle this gracefully instead of asserting.

Also remove an assert that wasn't really checking anything in
CreateZero. type.isScalar() || type.isVector() || type.isMatrix() can
only be false in case of a struct, and struct type was being checked
in the condition on the line above.

BUG=chromium:717385
TEST=angle_unittests

Change-Id: Iff0811d18d399d7b32b2b46deea5df172412eb8c
Reviewed-on: https://chromium-review.googlesource.com/492887Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>