sna: Add a pair of asserts to track down a NULL pointer dereference

Looks like the assumption for the location of the data is invalid,
allocation failure, perhaps?

References: https://bugs.freedesktop.org/show_bug.cgi?id=47597
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/sna_render.c

@@ -460,6 +460,8 @@ static struct kgem_bo *upload(struct sna *sna,
 			priv->mapped = false;
 		}
 		if (pixmap->devPrivate.ptr == NULL) {
+			assert(priv->ptr);
+			assert(priv->stride);
 			pixmap->devPrivate.ptr = priv->ptr;
 			pixmap->devKind = priv->stride;
 		}