tnl: Don't dereference NULL obj pointer in t_rebase_prims

tnl: Don't dereference NULL obj pointer in t_rebase_prims

Structurally the code is now similar to the handling of other
gl_buffer_object::obj pointers elsewhere in TNL.  The fixes tag is a
little bit misleading.  I think the change in that commit just exposes a
previously existing bug.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2746
Fixes: f3cce7087a5 ("mesa: don't ever bind NullBufferObj for glBindBuffer targets")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4512>