Fix a crash when parsing a shader with incorrect constructor arguments.

BUG=angleproject:911
TEST=WebGL conformance tests

Change-Id: I3459612f36185db861a361fa916cc6b671795037
Reviewed-on: https://chromium-review.googlesource.com/255870Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Zhenyao Mo <zmo@chromium.org>