Fixed Vertex Array Objects

Fixed Vertex Array Objects

Fixed glBindVertexArray. The early return was wrong,
binding vertex array 0 is perfectly valid.

All related dEQP tests pass.

Change-Id: Idd0ae745dc15877afb7c7323aab8253fb71e9017
Reviewed-on: https://swiftshader-review.googlesource.com/13549
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>