mesa: flip current tf object back to default if current is being deleted

In the rather unusual case of Bind + Delete, we need to make sure that
we unbind the current tf object.

Fixes dEQP-GLES3.functional.lifetime.delete_bound.transform_feedback

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Matt Turner <mattst88@gmail.com>
1 file changed