st/dri: fix a crash in server_wait_sync

Ported from i965 including the comment.

This fixes:
    dEQP-EGL.functional.reusable_sync.valid.wait_server

Cc: 18.1 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
(cherry picked from commit 0eaf0696)