vbo: Don't set node->min_index = max_index = indices_offset when merging

vbo: Don't set node->min_index = max_index = indices_offset when merging

I'm can't see why this is necessary.  There are already new fields
(node->merged.{min,max}_index) for the new values in the merged case.
But in vbo_save_draw.c, in the !draw_using_merged_prim case, we would
try and use the original node...with the now destroyed min/max index.

Fixes some assert failures when running with swtnl and forcing the
non-merged path (though it takes the merged path by default).

Fixes: 4c751ad67aa ("vbo/dlist: use a shared index buffer")
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8522>