aux/vbuf: prevent uint underflow and assert if no vbs are dirty

if this mask is 0, there is nothing to do here

Fixes: e73bf3b805d ("gallium: add start_slot parameter to set_vertex_buffers")

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10929>
1 file changed