If an FBO only has a depth/stencil attachment, the attachment index will
be 0, but that value was incorrectly being used as a flag for no
depth/stencil attachment.

Bug: angleproject:2361
Change-Id: I4ee803a392a29f7b261bc0cfabf9f2507dc7b178
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1541670
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>