Fix null pointer dereference in redeclaration error message 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/603368 Reviewed-by: Corentin Wallez <cwallez@chromium.org>