In some cases we would have a 'max enabled' attrib that was checked
incorrectly, and this could lead to edge cases which crash when we
have a vertex attribute and buffer both NULL.

BUG=angleproject:1081
TEST=dEQP-GLES3.functional.lifetime.attach.deleted_input.buffer_vertex_array

Change-Id: I62a9fa79f38340c99a708e54751d2b67dd4b6c1f
Reviewed-on: https://chromium-review.googlesource.com/286854Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>