drm/nouveau: Work around a crash during boot if noaccel is set.

index b244d99..c7ffa63 100644
@@ -650,6 +650,12 @@ nv50_display_vblank_crtc_handler(struct drm_device *dev, int crtc)
 	struct nouveau_software_priv *psw = nv_engine(dev, NVOBJ_ENGINE_SW);
 	struct nouveau_software_chan *pch, *tmp;
 
+        if (!psw) {
+                WARN_ON_ONCE(1);
+                printk(KERN_ERR "NULL software engine\n");
+                return;
+        }
+
 	list_for_each_entry_safe(pch, tmp, &psw->vblank, vblank.list) {
 		if (pch->vblank.head != crtc)
 			continue;