mesa: add check for NV_texture_barrier in _mesa_TextureBarrierNV()

mesa: add check for NV_texture_barrier in _mesa_TextureBarrierNV()

If an app called glTextureBarrierNV() without checking if the
extension was available, we'd crash with some gallium drivers
in st_TextureBarrier() because the pipe_context::texture_barrier()
pointer was NULL.

Generate GL_INVALID_OPERATION instead.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>