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>