When a function parameter name conflicts with another, the pointer
returned to ParseContext will be null.

BUG=chromium:745242
TEST=angle_unittests

Change-Id: Ie53bb06b0c6660e382d85aeda41f3a1b7df5a917
Reviewed-on: https://chromium-review.googlesource.com/603368Reviewed-by: Corentin Wallez <cwallez@chromium.org>