Silently ignore attempts to delete default VAO.

Silently ignore attempts to delete default VAO.

The spec for glDeleteVertexArrays states that "Unused names in arrays
are silently ignored, as is the value zero."

b/116699321

Change-Id: I7cca0336dd9841b360f8fa20f6c0ac9677ec3ce1
Reviewed-on: https://swiftshader-review.googlesource.com/21048
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>