FramebufferGL: fix a crash when there is no color buffer. BUG=chromium:675819 Change-Id: I7d143ba51bb16f28b145aa2b0410929d29e0b464 Reviewed-on: https://chromium-review.googlesource.com/422493 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>