gallium/auxiliary: Handle count == 0 in u_vbuf_get_minmax_index_mapped

gallium/auxiliary: Handle count == 0 in u_vbuf_get_minmax_index_mapped

This makes u_vbuf_get_minmax_index_mapped return min = 0 / max = 0
when info->count == 0.

That should never happen anyway, but this commit makes it at least
return a sane value that callers expect, and also allows us - and
GCC - to assume count != 0 for optimization purposes.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3050>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3050>