A va_list is undefined after it has been used by vsnprtinf. This was
causing crashes in the GL backend when the driver was returning big
info logs.

BUG=chromium:668223

Change-Id: I444194ecce2846960c8a27f20f322f7099c651e5
Reviewed-on: https://chromium-review.googlesource.com/421271Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>