llvmpipe: only read 0 for channels being read
KHR-GL45.robust_buffer_access_behavior.vertex_buffer_objects on the CTS 4.6.0 branch and this fixes it. Roland identified that if the vertex format doesn't contain channels then we shouldn't be overriding them to 0, so RGB fetch out of bounds should return 0 for RGB, but the A channel should still be getting back 1. Cc: <mesa-stable@lists.freedesktop.org> Reviewed-by:Roland Scheidegger <sroland@vmware.com> Part-of: <mesa/mesa!6287>