ES3: Fix off-by-one max enabled attrib.
ES3: Fix off-by-one max enabled attrib.

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/286854
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

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