Vertex Array Object crash fix

Vertex array objects are not allocated when generated,
but only when bound, so it's legal to have a null
vertex array object internally.

Change-Id: Ib28a388939e285425c09cfbc9f4efef3f4a8cead
Reviewed-on: https://swiftshader-review.googlesource.com/4700
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>