i965: allow unsourced enabled VAO
i965: allow unsourced enabled VAO

The GL 4.5 spec says:
    "If any enabled array’s buffer binding is zero when DrawArrays
    or one of the other drawing commands defined in section 10.4 is
    called, the result is undefined."

This commits avoids crashing the code, which is not a very good
"undefined result".

This fixes spec/!opengl 3.1/vao-broken-attrib piglit test.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler