dri2: SourceOffloads may be for DRI3 only

As a DDX may declare offload support without supporting DRI2
(because it is using an alternative acceleration mechanism like DRI3),
when iterating the list of offload_source Screens to find a matching
DRI2 provider we need to check before assuming it is DRI2 capable.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88514
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

hw/xfree86/dri2/dri2.c

@@ -156,6 +156,9 @@ GetScreenPrime(ScreenPtr master, int prime_id)
         DRI2ScreenPtr ds;
 
         ds = DRI2GetScreen(slave);
+        if (ds == NULL)
+            continue;
+
         if (ds->prime_id == prime_id)
             return slave;
     }