Fix some default return values in auto-gen. We don't generally test for the default return values of function calls that have validation errors, but the auto-gen script might not have been returning things correctly. For GLint values, default to returning -1 instead of zero. BUG=angleproject:1309 Change-Id: I736b5a7ef9b50ca1509087fe933c4a4e526531d8 Reviewed-on: https://chromium-review.googlesource.com/636522 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>