DRI2: Avoid a NULL pointer dereference

Bugzilla:  https://bugs.freedesktop.org/show_bug.cgi?id=41211

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jeremy Huddleston <jeremyhu@apple.com>

hw/xfree86/dri2/dri2.c

@@ -816,7 +816,8 @@ DRI2WaitSwap(ClientPtr client, DrawablePtr pDrawable)
     /* If we're currently waiting for a swap on this drawable, reset
      * the request and suspend the client.  We only support one
      * blocked client per drawable. */
-    if ((pPriv->swapsPending) &&
+    if (pPriv &&
+	pPriv->swapsPending &&
 	pPriv->blockedClient == NULL) {
 	ResetCurrentRequest(client);
 	client->sequence--;