i965: Make sure we always compute valid index bounds before drawing.

When doing software rendering (i.e. rendering to the selection buffer) we need
to make sure that we have valid index bounds before calling _tnl_draw_prims(),
otherwise we can crash.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=59455
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_draw.c

@@ -554,7 +554,8 @@ void brw_draw_prims( struct gl_context *ctx,
     * get the minimum and maximum of their index buffer so we know what range
     * to upload.
     */
-   if (!vbo_all_varyings_in_vbos(arrays) && !index_bounds_valid) {
+   if (!index_bounds_valid &&
+       (ctx->RenderMode != GL_RENDER || !vbo_all_varyings_in_vbos(arrays))) {
       perf_debug("Scanning index buffer to compute index buffer bounds.  "
                  "Use glDrawRangeElements() to avoid this.\n");
       vbo_get_minmax_indices(ctx, prims, ib, &min_index, &max_index, nr_prims);