radv/gfx10: fix the out-of-bounds check for vertex descriptors

When stride is 0, it should check against the offset not the index.

This fixes black character models with Beat Saber and missing snow
with Dragon Quest.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2233
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1975


Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3147>
(cherry picked from commit f3cccd05)