Fix clearing out-of-range default FB drawbuffer The default framebuffer is initialized with just one drawbuffer slot so the number of drawbuffers needs to be validated before checking if a buffer is attached. BUG=angleproject:2831 TEST=angle_end2end_tests Change-Id: I960c39357853d1cd4575b06a992cff33223ab3df Reviewed-on: https://chromium-review.googlesource.com/c/1264518 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>