mesa: don't use genned but unnamed xfb objects.

mesa: don't use genned but unnamed xfb objects.

If we try to draw or query an XFB object that hasn't been bound,
we shouldn't return any information.

This fixes a couple if cases in:
GL33-CTS.transform_feedback.api_errors_test

The ObjectLabel test is inspired by another test.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>